BEIRUT — Residents of Homs on Monday described scenes of panic and carnage in the city of central Syria undergoing a fierce assault by government troops, urging outside powers to intervene to halt the bloodshed.

"What is happening is indescribable," said Wael, a resident reached by telephone. "Security forces have been relentlessly shelling the city since this morning.

"On the streets, there are only bodies and wounded people along with tanks," he said. "There are tanks everywhere.

"People are in a state of panic and they are screaming, 'May God help us' or 'Where are the Arabs?'" he added. "We feel like the world has abandoned us."

Activists said the latest government assault on the city began at dawn with troops raining mortars and rockets on several neighbourhoods.

"We cannot venture outside because of the intensity of the shelling," said Saher, another resident. "The shells are coming down like rain."

The Syrian Observatory for Human Rights said at least 29 civilians were killed and dozens more wounded.

But it added the death toll was likely to rise because many of the wounded had suffered critical injuries.

The shelling, which began at dawn, was mainly targeting the neighbourhoods of Khaldiyeh, Baba Amro, Inshaat and Bab Sbaa, the Britain-based group said.

Syrian state television accused "armed gangs" of being behind the latest violence in Homs, which has become a flashpoint of the near 11-month revolt that has killed at least 6,000 people across the country according to rights groups.

A resident of the battered city told AFP that Monday's assault was the most intense since the outbreak of the unrest in mid-March.

"What is happening is horrible, it's beyond belief," said Omar Shaker, an activist also reached by telephone. The sound of gunfire and loud explosions could be heard in the background as he spoke.

Shaker said activists were transporting the wounded to the city's mosques for treatment.

"There is nowhere to take shelter, nowhere to hide," he said. "We are running short on medical supplies and we are only able to provide basic treatment to the injured."

Residents said hospitals had also run out of blood supplies.

Footage shot by a BBC undercover team in Homs showed buildings ablaze in rebel neighbourhoods as regime forces pounded them with heavy weapons.

Plumes of white smoke billowed into the sky.

Chants of "Allahu Akbar" (God is Greatest) could be heard from balconies and rooftops as residents put on a show of defiance in the face of the onslaught.

A video posted by activists on YouTube apparently showed a field hospital hit by the shelling in Baba Amro, one of the main targets of Monday's assault.

Wounded patients could be seen laying on stretchers on the floor amid pools of blood and shattered glass. Bodies of dead people with horrific injuries were also shown inside the hospital.

The authenticity of the video could not immediately be verified.

"Let the Arabs and Muslims come and see this, this is a result of their inaction," said Mohamed Mohamed, who identified himself as a doctor at the hospital.

Wael said some residents were scared of leaving the city while others, especially those with children, were braving sniper fire and fleeing.

"We want the international community ... to get rid of (President Bashar) al-Assad," he said. "Assad thinks he is going to scare people but they are going to rise up more and are more determined than ever."