"We ... request the Human Rights Council to hold an urgent debate on the deteriorating situation of human rights in the Syrian Arab Republic, and the recent killings in Al Qusayr," the ambassadors of the three countries wrote in their request to council president Remigiusz Henczel.

The UN human rights council is due to open its next three-week session on Monday, and the three countries called for the urgent debate to be held during the first week, lamenting "the escalating grave human rights violations" in Syria.

Council spokesman Rolando Gomez told AFP that president Henczel and other administrators would now examine a possible date for the debate, adding that Tuesday or Wednesday looked likely.

This would not be the first time the UNs top rights body meets to discuss the spiralling violence in Syria.

The council has previously held one urgent debate on Syria and four special sessions outside its usual four annual meetings and is already set to hear, during the coming session, a report from UN investigators into the human rights situation in the war-torn country.

That report will be presented on June 3.

The request for an urgent debate on the situation in Syria comes after the Friends of Syria group of governments that back the rebels met in Amman this week as part of the diplomatic efforts to convene a peace conference in Geneva next month.

The conference, dubbed Geneva 2, is intended to build on a never-implemented deal agreed in the Swiss city last year that called for a halt to the violence and a transitional government.

Since then, the conflict has spiralled ever more out of control.

More than 90,000 people have been killed and over 1.5 million Syrians have fled to neighbouring countries since the conflict began in March 2011.

Another 6.8 million people are in need of assistance inside Syria, including nearly 4.3 million people who have been displaced from their homes.

Overall, around 38 per cent of the countrys pre-war population of 22.5 million is in need of humanitarian assistance.

The regime assault on the rebel stronghold of Qusayr, in central Homs province, that began Sunday has meanwhile left more than 100 people dead, while thousands of civilians are believed to be trapped in the town.

The town, which lies near the border with Lebanon, is a key prize for the rebels, a conduit through which weapons and fighters can be channelled from Lebanon.

Qusayr is also important for Assads forces because of its strategic location between Damascus and the Mediterranean coast, the rear base for the regime.

The UN refugee agency said Friday it was hearing reports that all civilians left in the city, that used to count around 25,000 inhabitants, had been moved into one section of the town.

They "are pretty much trapped," said the UNHCR.

UNHCR spokeswoman Melissa Fleming said the UN agency was bracing for a possible flood of people from the town across the border into Lebanon.

But she added that so far many of those who had fled appeared to be having difficulty making it to the border due to "very high security risks".

She also said heavy fighting near the Jordanian border was likely the main reason why the number of refugees flooding into that country had plummeted from an average of 2,000 a day to basically zero in the past seven days.

"UNHCR is concerned about reports that many Syrians trying to flee may be backed up at the borders in areas that are extremely dangerous," she told reporters in Geneva.