President Obama has accused Syrian government forces of responsibility for "outrageous" bloodshed and called again for Bashar al-Assad to step down, as troops sealed off a rebel stronghold in the city of Homs and launched a bombardment using tanks, helicopters and artillery.

Speaking after a White House meeting with the Italian prime minister, Mario Monti, Obama said: "We both have a great interest in ending the outrageous bloodshed that we've seen and see a transition from the current government that has been assaulting its people."

His comments came as the international community struggles to find a common voice with which to confront President Assad.

Eyewitnesses said roads in and out of the besieged suburb of Baba Amr, in the south-east of Homs, were blocked, preventing the evacuation of children and the wounded, and food, water and medicine were running out.

The international community appeared to flounder over a coherent response. The UN secretary-general, Ban Ki-moon, condemned the Russian and Chinese veto of a security council resolution on the crisis over the weekend as "disastrous for the Syrian people". He said the failure to agree on collective action had "encouraged the Syrian government to step up its war on its own people".

The UN and Arab League have proposed a joint observer mission, while talks continued over the formation of an ad hoc "friends of the Syrian people" group to put pressure on the Assad regime without help from Moscow and Beijing.

Speaking at an international gathering in Sweden, the British prime minister, David Cameron, said: "It is quite clear this is a regime hell-bent on killing, murdering and maiming its own citizens … we need to take the toughest possible response we can."

But the options Cameron listed reflected the cautious, incremental approach the UK and other western governments have pursued after the security council debacle. "We also need to work with the [Syrian] opposition to try and help shape their future and assist them in whatever way we can. We also need to put together the strongest possible contact group of like-minded nations," he said.

The foreign secretary, William Hague, said there were no plans to arm Syrian rebels. He would not guarantee that Britain would not become involved in military action, but stressed: "We are clearly not planning military intervention."

Amid speculation the UK could assist the rebels with weapons or other equipment, Hague told Sky News: "Britain is not engaged in that and we haven't done that in any of the conflicts or we certainly don't have any plans to do such.

"We are intensifying our contacts with opposition groups, opposition groups mainly outside Syria. We're also increasing our support for organisations that get food and medical supplies in to people so badly affected by this situation."

In the absence of international consensus, there was no sign of any decisive action that might stop the worsening bloodshed in Syria.

More than 100 people were reported dead in Homs on Thursday during heavy bombardment by government forces, but that figure could not be independently confirmed in the absence of observers or humanitarian organisations. A local resident, Basil Abu Fouad, said it was impossible to estimate casualties accurately.

"We can't count the number of the dead in the rubble. When we pull someone from the rubble, we don't know if they were killed today, yesterday or before," Abu Fouad said by phone from a basement in Baba Amr.

"They are using helicopters and mortars and tanks, T-72 [former Soviet] tanks. Hundreds of homes have been demolished and they have come down on the heads of their owners.

"Communications have been completely cut off between neighbourhoods. The army have blocked access to the city. Some people tried to escape but they found all the roads were closed. There is no food left in the city. We don't have milk. All the water tanks have been targeted. We don't have medicines. If you go to the shops and try to get in, the snipers up on the roofs will shoot you," he said.

"The children will die here. All the people want is to escape. They can smash this place if they want. We just want to get out of there. But they won't allow us."

Another Homs resident, Abu Karam, watching from the nearby Karm al-Zeytoun district, said: "From my roof I can see rising smoke from Baba Amr. It is surrounded. They can't get anything in or out."

Another man from Baba Amr, who said he was Lieutenant Omar of the Free Syrian Army, said the rebels were powerless to fight back against rocket and shell fire: "All we do is to pull people from the wreckage."

Dani Abdul Dayan, a resident of the Insha'at, next to Baba Amr, claimed the army had taken over the Hikma hospital, expelled doctors and was using the patients as human shields. The claim could not be verified.

Writing in the Guardian, the Chinese ambassador to the UK, Liu Xiaoming, defended Beijing's decision to veto the UN resolution setting an ultimatum for Assad to step down. "Chinese people abhor the violence and bloodshed in Syria as much as those in Britain and other countries. China voted against the resolution for a simple reason: the resolution as drafted will not help cool down the situation. It does not facilitate political dialogue. It does not address distrust, or bring peace and stability to the region," he said.

In Homs, Abu Fouad said the security council veto had given the regime a green light to commit genocide against the Syrian people. "The whole world is against us. Nobody is helping us," he said, before the electricity failed. "There is nothing left but to pray."