Residents of a Syrian town besieged by President Bashar al-Assad's forces appealed to the world to "save us from death" in an open letter describing desperate conditions and suffering.

Hundreds of men, women and children in Mouadamiya had died and thousands had been wounded, they said.

Mouadamiya, on the southwest outskirts of the capital Damascus, was occupied by anti-Assad rebels last year and the government has been trying to win it back since then.

"For nearly one year, the city of Mouadamiya has been under siege with no access to food, electricity, medicine, communications, and fuel," said the letter, distributed by the opposition Syrian National Council on Monday.

"We have been hit by rockets, artillery shells, napalm, white phosphorous, and chemical weapons," it said.

The writers, who did not give their names, said they had managed to find enough power to run a computer and connect to the internet to send the letter.

The SNC said nearly 12,000 people face starvation and death in Mouadamiya. About 90 percent of Mouadamiya has been destroyed, few doctors remained, and residents were eating "leaves of trees".

Reuters cannot confirm reports from the besieged town due to government restrictions. The government says the residents of Mouadamiya are being "held hostage" by terrorists, the term it uses for armed opposition groups. It denies using chemical weapons.

United Nations humanitarian chief Valerie Amos said last week that despite the government evacuating 3,000 people this month, thousands more remain trapped inside Mouadamiya. Local doctors have told Reuters that hunger has become severe in recent months.

Juliette Touma, Syria spokeswoman for the United Nations Children's Fund, said their aid teams had been denied access several times to Mouadamiya.

"We know that there are children that are trapped in Mouadamiya. Exactly how many children, we don't know," she said.

"Mouadamiya is one part of a bigger story."

She said it was one of many areas that are "sealed off" to civilian movement by the warring parties.

RENEWED BOMBING

The Syrian Observatory for Human Rights monitoring organisation said on Monday that rebels and government forces clashed on the edges of Mouadamiya overnight and the army bombed the town.

"We appeal to your sense of humanity not to forget us," the residents' letter said. "We implore you to deliver our message to the whole world.

"Save us from death. Save us from the hell of Assad's killing machine."

More than 100,000 people have died during the war, which started with peaceful protests against four decades of Assad family rule in March 2011 then escalated into a civil war with sectarian overtones.

Western powers have mostly backed opposition forces while Russia and Iran support Assad. Moscow and Washington are planning to hold peace talks in Geneva next month but the warring parties have not expressed a willingness to compromise.