The UN's human rights chief said today the situation in Syria had deteriorated rapidly in recent weeks and demanded an immediate humanitarian ceasefire.

Navi Pillay said her office had received reports that Syrian military and security forces "have launched massive campaigns of arrest" and launched an onslaught against government opponents that has deprived many civilians of food, water and medical supplies.

Ms Pillay told an urgent meeting of the UN Human Rights Council that "hundreds of people have reportedly been killed since the start of this latest assault in the beginning of February 2012".

She called on Syria to end all fighting, allow international monitors to enter the country and give unhindered access to aid agencies.

Meanwhile, activists say troops have resumed heavy shelling of towns and cities in Syria's restive central region, a day after reports of 144 more people killed.

The British-based Syrian Observatory for Human Rights said shelling of the central town of Halfaya today killed at least four civilians and wounded dozens, many of them seriously.

The Local Coordination Committees, another activist group, said 20 people were killed and 100 wounded in the town.

Both groups said the rebel-held district of Baba Amr in the central city of Homs was under intense shelling. The LCC said 12 people were killed in Homs.

Yesterday, the LCC said 144 people were killed across Syria, scores of them in Baba Amr by security forces as they tried to flee.

Earlier, an aid group linked to the Red Cross said it had entered Baba Amr.

The International Committee of the Red Cross said a team from its Syrian sister organisation, the Syrian Arab Red Crescent, was trying to deliver medical supplies and evacuate wounded people.

Aid groups have been unable to enter Baba Amr since Friday, when 27 sick and wounded people were evacuated.

Ms Pillay's appeal prompted a bitter response from Syria's ambassador to the United Nations in Geneva, who accused the 47-nation council of promoting terrorism in his country.

Before walking out of the room, Fayssal al-Hamwi said today's meeting would only prolong the crisis in Syria.

"The call for holding the session is part of a pre-established plan," he said. "It is aimed at attacking the Syrian state and its institutions under the pretext of humanitarian needs."

A senior US diplomat said the time had come for nations to stop all financial and material support to President Bashar Assad's regime - a dig at Russia, which has long sold arms to Damascus and together with China has repeatedly used its Security Council veto to block international action on Syria.

"None can deny that Bashar al-Assad and his criminal cohorts are waging a brutal campaign of slaughter, bombardment, torture and arrest that already has murdered thousands of women, men and children, with more killed each day," said Esther Brimmer, the US Assistant Secretary of State for International Organisation Affairs.

Meanwhile, Syrian rebels smuggled a wounded British journalist out of the besieged central city of Homs today and whisked him to safety in neighbouring Lebanon.

They did not manage to evacuate a wounded French journalist or the bodies of two other Western reporters killed last week, activist groups said.

The Syrian opposition group Local Coordination Committees and global activist group Avaaz said photographer Paul Conroy was the only foreign journalist to escape Syria. Rima Fleihan, an LCC spokeswoman, said Mr Conroy was smuggled out by Syrian army defectors.

The LCC said other Western journalists are negotiating with the Syrian Arab Red Crescent to be allowed to leave Syria without having their videos and photos confiscated by authorities.

Mr Conroy's wire Kate said: "I have heard that he is out.

"All I can say is that we are delighted and overjoyed at the news, but I am not going to say any more than that at this point."

French President Nicolas Sarkozy later said an injured French journalist had been evacuated from Syria following tough negotiations.

Mr Sarkozy said Edith Bouvier suffered "multiple fractures" in an attack and that he is "glad that this nightmare is over".

Ms Bouvier works for the daily Le Figaro and was covering the Syrian regime's bloody crackdown on the anti-government movement.

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