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 neighbourhoods 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."

His comments came as Russia's foreign minister, Sergei Lavrov, held talks in Damascus on Tuesday with Syria's president, Bashar al-Assad, after Russia and China vetoed a UN security council resolution on Saturday that was designed to stop the bloodshed. The vetoes prompted global condemnation, with the US closing its embassy on Damascus on Monday, and Britain recalling its ambassador for consultations. On Tuesday the diplomatic exodus from Damascus continued, with France and Italy withdrawing their ambassadors. Six Gulf states – Bahrain, Kuwait, Oman, Qatar, Saudi Arabia and the United Arab Emirates – also pulled their envoys out and expelled Syrian ambassadors from their own countries.

Speaking on Tuesday, Lavrov said Assad had assured him he was "completely committed to the task of stopping violence regardless of where it may come from".

But the claim bore little resemblance to the bloody reality inside Homs.

Rabea described how government forces began a pincer movement against the city 10 days ago. He said soldiers slaughtered three families living on the edge of the Karm al-Zayton district, apparently to send a message to opposition forces who have been holding out against the regime since last year.

"It started with a massacre. Trucks of soldiers pulled up. They executed the women and men inside the house and stabbed the children with knives. They killed four members of the Bahader family, 11 from the A'kra family and six from the al-Muhammad family on 26 January," Rabea – who lives in the district – said. [The Guardian was unable to independently verify the report.]

Days later, [according to Rabea,] the bombardment began. First, the army forcibly evacuated students from Homs's university. It transformed the university's compound into a makeshift military base, bringing in truck-mounted missile launchers. The army also moved rocket launchers into the nearby town of Meskana, Rabea said.

"The regime changed its tactics. Instead of doing a ground incursion they are bombarding us from outside," he explained. "They are using artillery and land-to-air missiles." He added: "Many houses have been demolished. People were still inside them."

Other witnesses inside the city said there were ominous signs of preparations for a definitive final assault. Some 11 months after Syria's uprising began, with Homs its epicentre, Russian-made T-72 tanks had penetrated as far as Tripoli Street, south-east of Bab al-Amr, witnesses said. They counted 13 of them. Some 10-12 tanks surrounded al-Khaldiyeh. The Syrian army had entered via areas loyal to the regime, they added.

"We've heard continuous bombing since Monday. It hasn't stopped. Now there is firing as well," Waleed Fares, an activist speaking by satellite phone from inside al-Khaldiyeh said. "I've seen fighter planes and helicopters. We had 11 people killed here on Monday. Five people killed so far in my neighbourhood today. About 40-50 wounded. Women and children among the dead and injured."

Video footage filmed from a Homs rooftop, which emerged on Tuesday, apparently showed an apocalyptic scene: with missiles slamming every few seconds into residential areas, sending plumes of grey and black smoke into the skyline. The whine of a fighter jet could be heard. Against the sound of machine gun fire, a voice cries: "God is great." Other videos from Monday showed corpses lying in the corridor of a makeshift field hospital.

Another activist, Sufian, also speaking by satellite phone, said security forces had captured one hospital in al-Halemei. The injured were taken away to prison. The last field hospital in Bab al-Amr was bombed on Monday, he said. "We lost 10 people when we tried to evacuate it," he said. "This morning five people were killed. Since Friday more than 400 have died, and many more are under rubble."

He added: "We are using kitchen knives for surgery. All the field hospitals have been targeted. We are relying on domestic medicine cabinets to treat the injured. We don't have any blood for donations, or oxygen. We are calling on help from the whole world. We need urgent help opening the blockade of Homs."

Activists said the opposition fighters from the Free Syrian Army (FSA) were no match for their well-armed government adversaries, equipped with tanks, fighter planes, and Russian heavy weapons. "The soldiers who defected from the army only have Kalashnikovs. How can you face a battalion with a Kalashnikov?" Sufian asked. "Tanks have been captured, but they were very exposed, and the regime could easily target them. It's a target you cannot hide."

Amid the horror, some activists expressed – remarkably – optimism. They said that despite the ongoing massacre – with Homs becoming Syria's bloody counterpart to the Libyan town of Misrata – they still expected Assad's regime to crumble. Rabea said that in the wake of this weekend's failure to find a diplomatic settlement the only way the world could stop the slaughter in Syria was to arm the FSA.

"The international community needs to give the FSA money. And weapons. We need the Red Cross here. We need a no-fly zone. And we need safe havens so that people can flee." Who did he blame for the situation in Homs? "We blame Russia and China mainly for all the killing happening in the city now." He added: "Our crime is that we wanted freedom. But what we got from the regime was this increase of killing that started a year ago."

For the moment, the existential hell that is Homs continues. Waleed said that families who had survived four days of bombardment were sitting on the ground floor of their houses, hoping and praying the attacks would stop. Children were asking their parents if they would survive, he said, and the parents were unable to answer. "We're waiting," Waleed said. "Waiting to see if we live or die."