WASHINGTON/BEIRUT (Reuters) - The United States put Syrian President Bashar al-Assad on notice on Monday that it believes he was responsible for using chemical weapons against civilians last week in what Secretary of State John Kerry called a "moral obscenity."
"President (Barack) Obama believes there must be accountability for those who would use the world's most heinous weapons against the world's most vulnerable people," Kerry said in the most forceful U.S. reaction yet to the August 21 attack.
Speaking after U.N. chemical weapons experts came under sniper fire on their way to investigate the scene of the attack, White House spokesman Jay Carney said the use of chemical weapons was undeniable and "there is very little doubt in our mind that the Syrian regime is culpable."
Kerry said Obama was consulting with allies before he decides on how to respond.
"What we saw in Syria last week should shock the conscience of the world," Kerry told reporters.
"The indiscriminate slaughter of civilians, the killing of women and children and innocent bystanders by chemical weapons is a moral obscenity. By any standard, it is inexcusable. And despite the excuses and equivocations that some have manufactured, it is undeniable."
Military chiefs from the United States and its European and Middle Eastern allies met in Jordan for what could be a council of war, should they decide to punish Assad, who has denied using chemical weapons and blamed rebels for staging such attacks.
Many hundreds of people died in Damascus suburbs in what appears to have been the worst chemical weapons attack since Iraqi leader Saddam Hussein fatally gassed thousands of Kurds in 1988.
U.N. investigators crossed the front line from the centre of the capital, which remains under Assad's control, to inspect the Mouadamiya suburb, one of at least four neighbourhoods hit by the poison gas before dawn last Wednesday.
The United Nations said one vehicle in its convoy was crippled by gunshots fired by "unidentified snipers." The team continued on after turning back for a replacement car.
Syrian state television blamed rebel "terrorists" for the shooting. The opposition blamed pro-Assad militiamen.
"I am with the team now," a doctor who uses the name Abu Karam told Reuters by telephone from Mouadamiya. "We are in the Rawda mosque and they are meeting with the wounded. Our medics and the inspectors are talking to the patients and taking samples from the victims now."
Wassim al-Ahmad, an opposition activist, said members of the Free Syrian Army umbrella rebel organisation and the opposition's Mouadamiya Local Council were accompanying the inspectors on their tour of the suburb.
"The inspectors are now examining victims being treated at a makeshift hospital in Mouadamiya and are taking blood samples from them," Ahmad said.
Video filmed at the site showed inspectors in black and blue body armour and blue U.N. helmets walking through a street as curious onlookers came up to watch.
They shook hands with men who appeared to be rebels wearing camouflage vests, and were accompanied by doctors and residents. The group descended into the basement of a building where they were told injured survivors were being treated to protect them from more shelling. Another video showed an inspector interviewing a patient and taking notes.
Activists say at least 80 people were killed in Mouadamiya when the district was hit with poison gas. Hundreds of people also were killed in three other rebel-held districts - Irbin, Ain Tarma and Jobar.
An opposition activist said a large crowd of people gathered to air their grievances to the U.N. inspectors, who planned to take samples from corpses.
The inspectors later returned to their hotel and, within an hour, residents reported the shelling of Mouadamiya had resumed.
The decision to proceed with the mission despite coming under attack thwarted an apparent attempt to halt the inspectors' work before it began.
"The first vehicle of the Chemical Weapons Investigation Team was deliberately shot at multiple times by unidentified snipers in the buffer zone area," the United Nations said in a statement. "It has to be stressed again that all sides need to extend their cooperation so that the team can safely carry out their important work."
The inspectors had been stuck in a downtown hotel since the attack, waiting five days for government permission to visit the scene a few miles away. They had arrived three days before the incident, with a mandate to investigate earlier reports of more limited chemical weapons use.
Kerry said Assad's decision to finally allow access was too late to be credible. "That is not the behaviour of a government that has nothing to hide," Kerry said, adding that Assad's forces had also destroyed evidence by shelling the area.
"Our sense of basic humanity is offended not only by this cowardly crime, but also by the cynical attempt to cover it up," Kerry said.
He said the U.N. inspectors could at most confirm that chemical weapons were used, not who used them, but that it was Assad's government that has such weapons and the means of delivering them. He said Washington had additional information on the attack that it would make known soon.
Washington and its allies say they worry that the time that has elapsed, and continuous shelling by Assad's forces of the affected areas, could make it impossible for the inspectors to collect evidence. The United Nations said Secretary-General Ban Ki-moon was confident the team could get the data it needs.
Speculation has been mounting that Western countries will order some kind of military response to an incident that took place a year after Obama declared the use of chemical weapons a "red line" that would require strong action.
With his international credibility seen increasingly on the line, Obama could opt for limited measures such as cruise missile strikes to punish Assad and seek to deter further chemical attacks, without dragging Washington deeper into the war. The United States has started a naval buildup in the region to be ready for Obama's decision.
In neighbouring Israel, citizens have been queuing up for gas masks in case Assad responds to a Western attack by firing on Israel, as Iraq's Saddam did in 1991.
With tensions rising over Syria, British Prime Minister David Cameron cut short a holiday to lead a top-level security meeting. Obama, Cameron and French President Francois Hollande all spoke to each other and other allies in the past few days in a flurry of phone calls. Cameron also called Russian President Vladimir Putin on Monday.
Several NATO countries have issued statements pledging a response, although none has been specific about what is planned.
Top military officers of the United States, Britain, France, other NATO allies and the main anti-Assad countries in the region, including Saudi Arabia and Turkey, met in Jordan on Monday to discuss Syria, diplomats there said.
The conference was planned but took on new significance because of the latest events, the diplomats said.
The chairman of the U.S. Joint Chiefs of Staff, Martin Dempsey, a co-host of the meeting with his Jordanian counterpart, has been one of the voices in Washington urging caution and emphasising the costs of a full-scale military intervention in a war in the heart of the Middle East.
Obama, who withdrew U.S. troops from Iraq and is winding down the conflict in Afghanistan, is reluctant to involve the United States in another war. A Reuters/Ipsos poll published on Saturday showed about 60 percent of Americans opposed U.S. military intervention, while only 9 percent thought Obama should act.
Assad denies the accusations that his forces used chemical weapons and said the United States would be defeated if it intervened in his country.
"Would any state use chemicals or any other weapons of mass destruction in a place where its own forces are concentrated? That would go against elementary logic," he told the Russian newspaper Izvestia. "Failure awaits the United States as in all previous wars it has unleashed, starting with Vietnam and up to the present day."
Russia, Assad's main arms supplier and diplomatic defender in the U.N. Security Council, says rebels may have been behind the chemical attack. Foreign Minister Sergei Lavrov said any intervention in Syria without a Security Council resolution would be a grave violation of international law.
French Foreign Minister Laurent Fabius noted that Russia and China would probably veto a U.N. Security Council vote to allow strikes against Syria. But British Foreign Secretary William Hague said it would still be possible to respond to a chemical weapons attack without the Security Council's permission.
There are precedents. In 1999 NATO attacked Serbia, a Russian ally, without a Security Council resolution, arguing that action was needed to protect civilians in Kosovo.
Turkey, a NATO ally and major backer of Syria's opposition, said it would join any international coalition even if a decision for action could not be reached at the United Nations.
Additional reporting by Mariam Karouny in Beirut, Khaled Yacoub Oweis in Amman, John Irish in Paris, Katya Golubkova in Moscow, Steve Holland and Paul Eckert in Washington.; Writing by Matt Spetalnick and Claudia Parsons; Editing by Bill Trott and Christopher Wilson