Refugees are fleeing Syria in ever greater numbers as the conflict between President Bashar al-Assad's forces and rebels intensifies, UN agencies have said, .

Former Syrian Prime Minister Riyad Hijab, who defected this month, arrived in Qatar earlier to discuss how to unify opposition efforts to hasten Mr Assad's downfall, his spokesman said.

Mr Hijab, a Sunni Muslim, is the most senior serving civilian official to desert Mr Assad.

More than 250 people, including 123 civilians, were killed in Syria on Thursday alone, according to the Syrian Observatory for Human Rights, a British-based opposition watchdog.

Reports within Syria cannot be verified due to restrictions on reporters there.

Turkey is taking the brunt of a swelling exodus of refugees, with 66,000 Syrians now sheltering there, the Turkish state disaster and emergency authority said.

Some 1,500 arrived from the rebel-held border town of Azaz after Mr Assad's air force bombed it on Wednesday, killing at least 35 people, Turkey's Dogan news agency reported. It said another 1,500 from the devastated town were thought to be on their way.

Fighting is continuing in the northern city of Aleppo as rebels battle for control of Syria's biggest city. Mr Assad's forces have turned increasingly to air power to hold back lightly-armed insurgents trying to seize territory elsewhere.

Turkey's state-run Anatolian news agency said 13 of 86 casualties brought from Aleppo and Azaz to a state hospital in the Turkish border province of Kilis had died from their wounds.

More than 170,000 Syrian refugees have been registered in Turkey, Jordan, Lebanon and Iraq, the UN refugee agency said.

Humanitarian conditions in Syria have deteriorated as fighting worsens, cutting off civilians from food supplies, health care and other assistance, UN agencies say.

Diplomats from the world's major powers, along with key Arab governments and Turkey, are to meet at the UN in New York later to discuss what to do after the failure of peace efforts led by UN-Arab League envoy Kofi Annan.

Algerian mediator Lakhdar Brahimi is taking over from Mr Annan, who resigned two weeks ago.

The last UN monitors are due to leave Damascus by 24 August, UN officials said.

"It is clear that both sides have chosen the path of war, open conflict, and the space for political dialogue and cessation of hostilities and mediation is very, very reduced at this point," said deputy UN peacekeeping chief Edmond Mulet.