Residents in Homs said on Wednesday that the noose was tightening around their besieged city, with the Syrian army carrying out a ferocious bombardment against the helpless civilians trapped inside.

At least 27 people were killed on Wednesday, with about 200 injured, 50 seriously, activists said, after unrelenting artillery attacks. Activists said the victims included a four-year-old girl, Salam al-A'raa, shot in the head, in the opposition-controlled suburb of Baba Amr.

"We are seriously dying here. It is really war," Waleed Farah told the Guardian, speaking via satellite phone. He said: "It isn't war between two armies. It's between the army and civilians. You hear the rockets and explosions. You feel you are at the front. The situation for civilians is pitiful."

Waleed said the situation had worsened over the past 24 hours, five days after the Syrian army began shelling rebel-held areas of Homs late last week. He said that as well as a massive bombardment, government troops had sealed off the neighborhood of al-Khaldiyeh, a crucial supply-point for bringing food and medicine into Baba Amr.

He said government snipers had shot dead four men who tried to drive a van into Baba Amr. They had been attempting to deliver bread. "The sniper situation has gone mad. They [government soldiers] know that all the supplies come from al-Khaldiyeh."

It was now impossible to evacuate the wounded and the dead from Baba Amr, he said, with volunteers who had gone there trapped.

Another activist, Raji, speaking from a basement inside Baba Amr, said Syrian forces were using different, heavier artillery rounds – with devastating effect. In addition to the 27 people killed on Wednesday, he said "many people" were lying dead under the rubble of their houses.