The fate of the besieged Baba Amr quarter in the Syrian city of Homs remains unclear after government troops launched a ground offensive on rebels.
Sources on both sides say government troops tried to advance on several fronts after weeks of heavy shelling.
Syrian officials said their forces were "mopping up" while activists said the attack had been repelled.
Meanwhile, UN humanitarian chief Valerie Amos says she has been denied entry into Syria.
In a statement, Ms Amos said the refusal came "despite my repeated requests to meet Syrian officials at the highest level to discuss the humanitarian situation and the need for unhindered access to the people affected by the violence".
She had been waiting in neighbouring Lebanon for the Syrian government's response to her application, UN diplomats said.
One of the two Western journalists still trapped in Homs is refusing to leave without injured Syrian civilians, say the Local Co-ordination Committees (LCC), a network of activists.
Edith Bouvier, who was seriously wounded last week in the bombardment of a makeshift media centre, is asking the French ambassador to come personally and to do whatever he can, the LCC say.
Earlier, the French government said it stood ready to evacuate Ms Bouvier and photojournalist William Daniels, who were on assignment for Le Figaro.
"We expect the government in Damascus to provide all the conditions for a safe and rapid evacuation, in particular an immediate ceasefire in Baba Amr," the French foreign ministry said in a statement.
A Spanish journalist trapped with them, Javier Espinosa, has now escaped to Lebanon.
All three of them, and wounded British photojournalist Paul Conroy, were in a convoy to smuggle them out of Syria when it came under fire from Syrian forces.
The convoy split up, and the two French journalists were escorted back to Baba Amr. Mr Conroy reached Lebanon on Tuesday and is now safe.
Campaign group Avaaz, which organised the escape mission, said 13 Syrians died trying to smuggle the Westerners out of the country.
"Javier Espinosa risked his own rescue when he was separated as he stopped to attend to wounded activists as they were shelled," said Avaaz founder Ricken Patel.
Two journalists were killed in the shelling last week.
Explosions echoed across Homs on Wednesday as the attack on Baba Amr began.
Officials in Damascus told reporters that clearing out the rebels would only take a few hours and that it had already become a mopping up operation.
But activist postings on the internet said the Free Syrian Army fighters had repelled repeated attacks by army forces and inflicted heavy losses.
The government has been reinforcing its troops around Baba Amr for some time.
Syrian activists said at least 23 people were killed across Syria on Wednesday.
These figures cannot be confirmed independently.
The BBC's Jim Muir, in neighbouring Lebanon, says reports are circulating that the government has brought up units of the feared Fourth Armoured Division under President Assad's brother Maher.
For both sides, the district has taken on symbolic significance, our correspondent says.
UK Foreign Secretary William Hague urged Syria to abandon plans for a ground offensive and allow in humanitarian agencies, saying the assault was "doomed to fail".
The US and France are leading efforts to draft a new UN Security Council resolution on the crisis, calling for an end to violence and access for aid.
However, the text also suggests that Mr Assad is responsible for the crisis, Western diplomats say.
One diplomat told Reuters the draft had been circulated among "a small circle of like-minded countries".
The text, which is still in its early stages, focuses on humanitarian aid for Syria, in the hope both China and Russia will back it.
They vetoed earlier Security Council resolutions in October and February, out of concerns that UN action could trigger Libya-style regime change.
China has indicated it is in favour of creating conditions for aid to be sent.
Foreign Minister Yang Jiechi told the head of the Arab League that China was willing to work with the Arab nations for ''a peaceful and proper settlement of the Syria issue'', China's official Xinhua news agency reported.
China has accused the West of pursuing "hegemonistic ambitions" in Syria under the guise of "humanitarian concern".
Correspondents say it remains far from clear whether Beijing will or will not veto any new Security Council resolution.
The Syrian government says at least 1,345 members of the security forces have been killed combating what it calls "armed gangs and terrorists", and puts the number of civilians killed at 2,493.