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 neighbourhood 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.

The Guardian has been unable to independently verify accounts from residents or casualty figures.

Raji said he and other activists had filmed government armoured personnel carriers entering Al-Insha'iat, an outlying part of Baba Amr, close to a mosque. "We took pictures and then we had to run away. We were scared for our lives," he said, adding: "The tanks have started to move inside our neighbourhood."

Elsewhere in Syria there appears to be growing evidence that amid diplomatic failure, and Russia and China's veto of a UN resolution to end the bloodshed in the country, Bashar al-Assad, Syria's president, has launched a conclusive military offensive against the rebels.

Activists in the former opposition stronghold of Zabadani, 20 miles north-west of Damascus, described how hundreds of tanks were pounding the town.

The opposition Free Syrian Army took control of Zabadani last month. Now, the Syrian army was on the brink of taking it back.

"There are about 300 tanks besieging the city from four positions. I saw so many tanks I couldn't count them," activist Fares Mohamad told the Guardian. He said the tanks had been firing shells since Saturday – with 18 people killed so far, and 300 with light injuries. More than 40 houses have been demolished, he said.

He added: "We have set up a field hospital in the basement of a house. But we don't have any medical equipment. There are also many people who have gone missing." Fares said that more than 1,200 families had fled to Bloudan, to the east of Zabadani, adding that the encircled town had no gas or fuel or communications.

The regime's tactics had been entirely cynical, he said, with the mountain town's fire station and hospital among the first targets, leaving nowhere to treat the injured.

Troops then began shelling residential districts. "The Free Syrian Army are in Zabadani but they can't stop the shelling. If you have mortars coming from tanks, what can you do?" he asked.

Syrian security forces also pounded residential neighbourhoods in the northern province of Idlib, the southern region of Daraa, and the Damascus suburb of Douma, activists said. In Homs, Syria's state-run television channel claimed gunmen had fired mortar rounds at the town's oil refinery and into Homs's Baath university – now used by Damascus as military camp, according to activists.

They said the Syrian army were attacking opposition enclaves from three fixed positions: the university, and the army and air force academies. Lightly armed Free Syrian Army troops were unable to fight back against an invisible adversary several kilometres away, they added. "When they [the army] come inside the city we will fight to our last breath," Raji said.

The medical situation inside Homs was desperate – with no proper facilities and the wounded treated in improvised field hospitals. In a gruesome video posted on Wednesday, one doctor from Baba Amr, Muhammad Al-Muhammad, said shooting had been going on since 5am, with more than 200 rockets fired within three hours.

He pointed to several victims who, he said, had been shot in the head. "We can't do anything for them. We are treating them in homes." In a desperate appeal, he said: "I call upon Erdogan in Turkey. I call upon Emir Hamad of Qatar. I call upon King Abdullah of Saudi Arabia. I call upon all the Muslims of the world to put pressure on Bashar, the monster."

The prospects for the civilians stuck in Homs appear bleak. Raji said there was no way in or out of the town, with residents sheltering in the ground floor or basements of houses. "I myself am trying to get my family out, especially my sisters. But the problem is that no one can get out," he said.

Raji said he and others had given up on the idea that the international community was coming to the rescue. He added there was perplexity that the world was taking its cue from Russia and China – which, he said, have in effect backed the Syrian president. "We don't believe Russia and China are more powerful than the whole world. And we don't understand why the world can't open a humanitarian corridor to help us."