Residents of Syria's third city report gunfire in a prison and hospital, and fear a full-scale assault as tanks mass outside

Syrian forces have stormed the country's third city, Homs, a day after at least 34 people were killed in earlier raids, as the troubled country's president defied calls by Barack Obama and European leaders to end the violence and resign.

Reinforcements were en route to the embattled city and residents there described a "terrible" night, with shooting resounding through empty streets and tanks again on the outskirts of town.

One Homs resident told the Guardian that shooting had been heard inside a hospital and a prison in the early hours of Saturday. Residents fear a full-scale assault may be imminent.

International calls for the president, Bashar al-Assad, to leave have sharply intensified scrutiny of his regime and its sustained crackdown against demonstrators, which it continues to cast as a fight against terrorists.

The hardened western stance is set to greatly increase the stakes for Assad, who now faces pariah status among leaders whose attention he had previously coveted.

It has also for the first time raised the possibility of a Libyan-style military intervention, something which had not previously been considered despite five months of violence in which an estimated 2,500 people have died and which have all but shut down the Syrian economy.

A Guardian poll published on Saturday revealed that 82.4% of respondents supported some sort of military intervention in Syria.

A state-owned Syrian newspaper described the calls by the US president – as well as the German chancellor, Angela Merkel, French president, Nicolas Sarkozy, and British prime minister, David Cameron – as the "face of the conspiracy" that it claims is being waged against it.

Assad has yet to respond to the western statements, which were delivered on Thursday. However, there are fears that the demands could embolden him, giving him little option but to fight as he struggles to retain control of the hardline police state his family has ruled as a personal fiefdom for more than four decades.

His traditional international support base remains resolute. Iranian support for the Assad regime makes it a key factor in calculations and Russia has said it does not support the call for Assad to leave.

Another one-time ally, Turkey, was on Sunday hosting a meeting of Syrian opposition groups who are attempting to elect a national council, as it too struggles to deal with the increasingly grave situation across its volatile border.

The new body is attempting to position itself as an alternative leadership, in the same way that the National Transitional Council did in the weeks after Colonel Gaddafi was ousted from eastern Libya. That body eventually won international recognition. However, Syria's nascent opposition has struggled to gain momentum.

A high-level United Nations team was due to arrive in Damascus on Sunday to assess the humanitarian situation across the country.

Assad has reportedly told the Office for the Co-ordination of Humanitarian Affairs that it can travel to any part of the country it wishes, including Latakia, which was besieged by land and sea during a major security operation earlier in the week.

Before the planned visit to Latakia, a western diplomat said reports had been received of a large-scale clean-up of the al-Ramel Palestinian refugee camp in the city, which was heavily targeted.

"Reports of a clean-up square perfectly with the version of events which the regime is denying," the diplomat said. "But any attempts to whitewash and destroy evidence can only backfire on this isolated regime.

"The evidence in the form of personal testimonies of what happened in Latakia is overwhelming and undeniable. Assad can run but he can't hide from the arm of international law which is closing in on him."

Residents of Hama and Homs reported similar clean-ups by government officials after rampages through both cities by security forces in recent months.

In Homs, the resident who spoke to the Guardian portrayed a city bracing itself for renewed destruction. "There are snipers on all the buildings. The tanks aren't in the centre, but around the edges. There is a general strike and all the shops are closed, nobody is opening. The situation is terrible – even after Assad says there aren't tanks and after Obama tells him to step aside."

The UN last week said it had identified 50 Syrian regime figures who may have committed crimes against humanity. In another sign of mounting international anger, the EU is considering placing a ban on Syrian oil exports, which account for 25% of the country's economy. With industry at a standstill, no tourism, and cash reserves rapidly dwindling, such a move would likely prove difficult for Assad's regime to withstand.