A siege of Syria's central city of Homs entered its 300th day on Sunday, as troops loyal to President Bashar Al-Assad wage a campaign to oust rebel groups holed up there, the opposition said.

"Three hundred days have gone by since the start of the siege of the heroic city of Homs, capital and beating heart of the Syrian revolution," the Syrian National Council, a key component of the main opposition National Coalition, said.

Homs, the country's third largest city, was one of the first to join the anti-Assad revolt that began two years ago with peaceful protests but morphed into a bloody insurgency after a fierce regime crackdown on dissent.

Today, some 80 percent of the city is under tight army control, and troops use tanks, helicopters and other warplanes to bombard besieged rebel enclaves.

The army and security forces began the siege in June 2012, setting up checkpoints all around the rebel-held districts.

Daily battles rage on the edges of the insurgent neighbourhoods, and on Sunday, the army pounded the districts of Khaldiyeh, Qarabis, Qussour and Juret al-Shiyah.

"Three hundred days have gone by while the world has looked on... with all kinds of war crimes and crimes against humanity being committed in this city," the SNC said in a statement.

"Schools and hospitals have been destroyed, water and electricity have been cut off, as have communications and food supply... Civilians are deprived of medicine and treatment," the opposition group added.

Activist Abu Bilal spoke to AFP via the Internet about life in a city under siege.

"This morning I attended the burial of a friend of mine, who had two children. When I was on my way back from the burial, I was told of the death of another friend. That's our daily reality. Every day there's death around us," he said from the besieged heart of Homs.

"A friend of mine, a rebel fighter, was abandoned by his fiancee because she could no longer wait for the siege to end. She married someone else. He says his heartbreak is worse than the siege itself," said Abu Bilal.

But some residents have found ways of coping.

"Some people are using their mobile phones to send photos to relatives who are outside the siege," he added.

The SNC, meanwhile, denounced "the policy of sectarian cleansing" practised by Assad's ruling minority Alawite clan against the Sunni population in Homs.

The majority of Syria's rebels are Sunnis, as is the population, and sectarian hatred has increased as the violence has grown steadily worse.

Homs is a strategic junction, linking Damascus to the majority Alawite coastline.