TUNIS/BERLIN (Reuters) - Battling tanks and snipers in Misrata's streets, the rebels holding the city are exploiting a defender's natural advantage in urban warfare to survive a two-month onslaught by better-armed government troops.
Blocking a main street with sand-filled trucks and trying to isolate and flush out gunmen loyal to Libyan leader Muammar Gaddafi firing at them from rooftops, insurgents are waging a desperate fight to cling on to their last major western bastion.
But Misrata's fate will likely depend on whether they can prevent their foes from seizing or denying access to its port -- a lifeline for trapped civilians and for food and medical aid, as well as a possible way to bring in arms and ammunition.
"Control of the port is essential because without that they would be truly cut off, they would fold, they would not be able to withstand the siege," Shashank Joshi, analyst at Britain's Royal United Service Institute, said.
Footage of rag-tag rebels with rocket-propelled grenades and automatic rifles crouching among crumbling buildings and launching hit-and-run attacks evoke images of other city sieges -- such as the Serb encirclement of Sarajevo in the early 1990s.
But analysts say they have a much-needed edge in fighting in built-up areas on their home turf. They know the terrain better than their adversaries, who may also experience difficulties in using their heavy weaponry to maximum effect.
"Even the most efficient and professional troops in the world -- like the Americans, the British, who have been in Iraq for example -- have found that in urban fighting the advantage is always on the defender," said military analyst Paul Beaver.
Insurgents can use their local knowledge to spring ambushes, and the only way to seize Misrata would be to start levelling it, as the Soviets did in Berlin at the end of World War Two.
"The Soviets destroyed whole neighbourhoods. There isn't the firepower in the Libyan army to do that," Beaver added.
But the same cityscape that gives the rebels cover and room for swift tactical manoeuvring also makes it hard for NATO warplanes to strike at government positions.
The rebels have complained bitterly about the military alliance's failure to take more decisive action against Gaddafi's forces and tip the balance in their favour, warning of an impending "massacre" in Misrata if it fails to do so.
"Unfortunately, NATO's mission did not succeed and they have clearly failed to protect civilians in Misrata," a rebel spokesman named Abdelsalam said, echoing the views of others.
A Reuters correspondent who travelled twice to Misrata on government-organised trips since fighting erupted there in late February said pro-Gaddafi troops controlled the southern section of Tripoli Street, a main thoroughfare and a key battle ground.
Gaddafi loyalists were trying to position as much military hardware as possible inside the city.
"I saw many anti-aircraft guns on the roofs of residential buildings, also underneath large trees," the correspondent, Maria Golovnina, added. She last went to the city's outskirts on April 9. Rebels have since claimed gains on Tripoli Street.
The head of NATO's military operations in Libya Monday accused forces loyal to Gaddafi of underhand tactics in Misrata.
"The regime's forces have used snipers on top of mosques, they are hiding beside hospitals, they have got their armoured vehicles in schools and, in fact, they have even taken their uniforms off," Lieutenant-General Charles Bouchard told the Canadian Broadcasting Corp.
The outcome of the battle for Misrata could have wider significance for the Libyan civil war, which has reached stalemate with inconclusive fighting on its fluid eastern front.
"If the government troops were able to take over Misrata it would give them a huge victory," Beaver said.
Using Russian-made Grad rockets and mortars, Gaddafi's forces have unleashed daily bombardments of rebel positions as well as residential areas, including one attack that killed several civilians in a bread queue last week, rights groups say.
Hundreds of people are believed to have been killed in what Western powers have condemned as a "medieval" siege to force Misrata's 300,000-strong population into submission.
But the insurgents say they are putting up stiff resistance, thwarting enemy attempts to advance on the port and further into Misrata's centre. They said they surrounded three loyalist-held buildings in the city Sunday, taking control of two of them.
"They know that once they step out of their tanks and armoured vehicles they will have to face rebel fighters in house-to-house battles," Abdelsalam, the spokesman, said.
It is hard for outsiders to get a clear picture of the situation in Misrata, which like many other Libyan cities rose up against Gaddafi's four-decade rule in February, and the rebels' claims are difficult to verify independently.
Libyan officials say they are fighting armed militia with ties to al Qaeda bent on destroying the country, denying that government troops are bombarding Misrata or shelling civilians.
Gaddafi's son Saif al-Islam said the army was rooting out terrorists hiding in the city, as the Russians did in the Chechen capital, Grozny, or the Americans in Fallujah in Iraq.
"It's exactly the same thing," he told the Washington Post. "I am not going to accept it, that the Libyan army killed civilians. This didn't happen. It will never happen."
But rebel and witness accounts paint of picture of indiscriminate shelling and intense fighting in the city and on its outskirts.
Buildings have been ripped apart by artillery fire, burnt-out vehicles lie in rubble-strewn streets, and doctors are treating wounded civilians with inadequate resources.
The insurgents say government forces are deployed on Nakl al Thaqeel road by the coast, on the outskirts of the eastern Kasr Ahmad residential area near the port and on or near Tripoli Street, which enters the city from the south.
They say Gaddafi forces are pounding districts close to the port with rockets and mortars. The shelling has closed Nakl al Thaqeel, used by heavy trucks to reach the harbour, forcing Misrata's insurgents, civilians and aid groups to use another.
"They are bombing residential areas day and night. It's non-stop," Ibrahim Ali, who arrived in Tunisia last Saturday on a boat from Misrata run by a medical charity, said.
Asked who was controlling most of the city, the 22-year-old said: "It's fifty, fifty. It can change quickly."
Additional reporting by Mariam Karouny in Beirut; Christian Lowe in Algiers; Marie-Louise Gumuchian in Tunis; Editing by Giles Elgood