Warnings from Syrian activists of a humanitarian catastrophe in Homs have grown more desperate as government forces resumed shelling an opposition stronghold in the city.

About 30 people, including two Western journalists, were killed in shelling on Wednesday - most of them in the rebel-held Baba Amr neighbourhood which is the centre of the resistance in the city. Homs has been under a fierce government attack for nearly three weeks now.

Homs-based activist Omar Shaker said intense barrages hit residential districts in Baba Amr again on Thursday, but there was no immediate word on casualties. He said food, water and medical supplies are running dangerously low.

"Every minute counts. People will soon start to collapse from lack of sleep and shortages in food," he said.

On Wednesday, shelling of Baba Amr killed American-born veteran war correspondent Marie Colvin and French photographer Remi Ochlik. They were among a group of journalists who had crossed into Syria illegally and were sharing accommodation with activists, raising speculation that government forces targeted the makeshift media centre where they were staying.

A Syrian foreign ministry spokesman offered condolences to the families of Ms Colvin and Mr Ochlik but rejected any responsibility for their deaths. The spokesman urged foreign journalists to respect Syrian laws and not to enter the country.

Foreign Secretary William Hague said nations were considering action against Syria in meetings on the sidelines of an international conference in London. Mr Hague said he would discuss "what we can achieve" with US Secretary of State Hillary Clinton and many of the Arab League leaders .

Meanwhile, a French journalist wounded in the shelling that killed two colleagues has pleaded with her government to evacuate her so she can have an operation.

In a video posted on YouTube, Edith Bouvier said her leg is broken in two places and that she has received some medical treatment but now needs an operation.

She was calm throughout the six-and-a-half minute video, even smiling.