Residents inside the besieged city of Homs claim they are under "genocidal attack" from a Syrian regime apparently deaf to international opinion and determined to "bomb, starve and shoot" them into submission.

On Tuesday night the city was under massive continuous bombardment, witnesses told the Guardian, with rockets raining down from the sky every few minutes, and helicopters and fighter planes circling overhead. They said Syrian army tanks had encircled opposition-held suburbs, in preparation for what they feared was a final, deadly ground assault.

"The regime didn't expect us to continue our struggle against them," activist Karam Abu Rabea said via Skype. "They didn't think we would persist. So now it is using its last card. It is the genocide card."

Rabea described the humanitarian situation as appalling. He said the regime was deliberately attempting to starve families trapped in rebel-controlled districts. Army snipers had been positioned on the main roads, he added, and were able to mow down anyone who moved on smaller, intersecting side roads. No one could escape, he said. Two journalists – Salah Murjan and Khalid Abu Salah, documenting the horrors of Homs – were shot by snipers.

Rabea said: "There is no food allowed to get inside neighborhoods opposing the regime. Especially bread. We don't have any bread. They are targeting the vital installations of the city: bakeries, the hospital, mosques. Some of the bakeries were shut by force. The regime cut off internet and phones on Monday. I have a satellite set, which is why I can speak to you. The Assad regime is trying to destroy Homs completely."