ISTANBUL (Reuters) - Turkish Prime Minister Tayyip Erdogan said on Thursday a Syrian passenger plane forced to land in Ankara was carrying Russian-made munitions destined for Syria's defence ministry, ratcheting up tensions with his country's war-torn neighbour.
Damascus said the plane was carrying legitimate cargo and described Turkey's actions as an act of "air piracy", while Moscow accused Ankara of endangering the lives of Russian passengers when it intercepted the jet late on Wednesday.
The grounding of the plane was another sign of Ankara's growing assertiveness over the crisis in Syria after Turkey's chief of staff warned on Wednesday the military would use greater force if Syrian shells continued to land in Turkey.
"This was munitions from the Russian equivalent of our Mechanical and Chemical Industry Corporation being sent to the Syrian Defence Ministry," Erdogan told a news conference in the capital Ankara, referring to a state-run Turkish manufacturer that supplies the country's army.
Russia's foreign ministry declined immediate comment but its arms export agency said earlier it had no cargo on the flight, while the Interfax news agency quoted Yelena Kara-Sal, a top Russian consular official, as saying the cargo seized by the Turkish authorities was not of Russian origin.
Syrian Air chief Ghaida Abdulatif told reporters in Damascus the plane was carrying civilian electrical equipment.
Turkey has become one of Syrian President Bashar al-Assad's harshest critics during an 18-month-old uprising that has killed some 30,000 people, providing sanctuary for rebel officers and pushing for a foreign-protected safe zone inside Syria.
Russia has stood behind Assad and an arms industry source said Moscow had not stopped its arms exports to Damascus.
Military jets escorted the Airbus A-320, which was carrying around 30 passengers, into Ankara airport after Turkey received an intelligence tip-off. The Turkish foreign ministry said the plane had been given a chance to turn back towards Russia while still over the Black Sea, but the pilot chose not to do so.
"This hostile and deplorable Turkish act is an additional indication of the hostile policy of Erdogan's government," Syria's foreign ministry said in a statement, accusing Ankara of "harbouring terrorists" and allowing them to infiltrate Syria.
Russian President Vladimir Putin had been expected to visit Turkey at the start of next week but Turkish officials said hours before the plane was grounded that Russia had requested the visit be postponed, citing his heavy work schedule.
Asked if the postponement was linked to the grounding, Putin's spokesman Dmitry Peskov said Putin and Erdogan had discussed a new date by phone on Monday, two days before the incident, and December 3 was likely. He did not comment further.
Turkey said it would stop more Syrian civilian aircraft using its airspace if necessary and instructed Turkish passenger planes to avoid Syrian airspace, saying it was no longer safe.
"We are determined to control weapons transfers to a regime that carries out such brutal massacres against civilians. It is unacceptable that such a transfer is made using our airspace," Foreign Minister Ahmet Davutoglu said.
Turkey has boosted its troop presence along the 900-km (560-mile) border and returned fire over the past week in response to shelling from northern Syria, where Assad's forces have been battling rebels who control swathes of territory.
"Turkey has crossed a new threshold," said former Turkish diplomat Sinan Ulgen, chairman of the Centre for Economic and Foreign Policy Studies think-tank.
"With the action they took last week the government is in the slightly more comfortable position of having shown it has the strength to retaliate."
Turkish Chief of Staff General Necdet Ozel said on Wednesday his troops would respond "with greater force" if the shelling continued and parliament last week authorised the deployment of troops outside Turkish territory.
Such approval has in the past been used for Turkish strikes against Kurdish militant bases in northern Iraq, where Turkey's last major incursion was in early 2008, when it sent 10,000 troops backed by air power over the border.
Some 25 fighter planes were sent to a military base in the southern city of Diyarbakir, around 100 km from the Syrian border, on Monday, the Dogan news agency said. Turkey has scrambled its F-16s to the Syrian border before, although air strikes inside Syria would be a major escalation.
Turkey has repeatedly made clear that beyond like-for-like retaliation it has no appetite for unilateral intervention in Syria. Such a move would be fraught with risks, as the row with Moscow over the grounded plane highlights.
Turkey relies on Russia, which has blocked tougher U.N. resolutions against Damascus, both for its domestic energy needs and to help it realise its greater ambitions as a hub for energy supplies to Europe.
Many Turks see Russia as harbouring sympathy towards the militant Kurdish Workers Party (PKK), which has stepped up violence in southeast Turkey in recent months. Turkish officials believe Syria and Iran have also been backing the group.
"We get 80 percent of our natural gas from Iran and Russia. Already the PKK card is being used by Iran against Turkey ... so the risks for Turkey of being involved in even a limited operation are huge," Ulgen said.
The establishment of foreign-protected safe zones in Syria would be hazardous, with the exit strategy for foreign forces dependent on the Syrian opposition's ability to topple Assad.
The Syrian rebels are outgunned by the government but can still strike at will, while Assad has assumed personal command of his forces, convinced he can prevail militarily.
Rebels attacked a Syrian army base near the main northern highway on Thursday to try to consolidate their control over the supply line to Aleppo, days after capturing a strategic town in the area, opposition activists said.
They used at least one tank seized from the army, as well as rocket-propelled grenades and mortar bombs, to hit the Wadi al-Deif base, three km (2 miles) east of the town of Maarat al-Nuaman, which they captured this week, they said.
Additional reporting by Dominic Evans in Beirut, Gulsen Solaker in Ankara, Ayla Jean Yackley in Istanbul, Jonathon Burch in Hatay and Thomas Grove and Steve Gutterman in Moscow; Editing by Michael Roddy; Writing by Nick Tattersall; Editing by Jon Boyle