Refugee crisis: Up to 20,000 Syrians massing at Bab al-Salam border crossing with Turkey, UN says

People have been pushed north as a Syrian army advance threatens to encircle Aleppo

Syrian refugees wait at the closed Bab al-Salam border crossing with Turkey (AFP)

Up to 20,000 Syrian refugees fleeing fighting in the north-western Aleppo province have gathered at a border crossing with Turkey, UN officials have said.

Turkey has been forced to shut the border for the second day in a row after being overwhelmed by the number of refugees fleeing an offensive by pro-Assad forces in the region in recent weeks.

The Turkish Prime Minister Ahmet Davutoglu said they would continue to feed and shelter the refugees but did not say when or if they would be allowed in.

On Wednesday, UN backed peace talks were “temporarily halted” after the army - backed by Russian air strikes - broke a three-year siege on government controlled villages in the region, Nubul and Zahraa, and disrupted a vital opposition supply line from Turkey to the city of Aleppo.

Aid agencies in southern Turkey are frantically trying to help the people massing at the Bab al-Salam border crossing following the most sustained assault on the area in the five years since the civil war began.

The advance has meant thousands are fleeing Aleppo - which had previously been a stronghold of the rebels - after a ground assault laid siege to the city to push the rebels out of the southern part they still control.

UN officials have said up to 20,000 Syrians are massing on the border (AFP)

Rebel groups in the area, including Jabhat al-Nusra, have said they could be forced out the next few days.

The deputy spokesman for the UN Secretary-General, Farham Haq, said the refugees were being temporarily housed in the towns of Azaz and Afrin in north-western Syria.

Nato’s Jens Stoltenberg criticised Russia for “undermining efforts to find a political solution to the conflict” by using air strikes to help President Assad rather than specifically targeting Isis.

Turkish President Recep Tayyip Erdogan said Russia must be held accountable for the civilians it had killed in Syria - saying between them Moscow and Damascus were responsible for the deaths of 400,000 people.

He accused the country of trying to set up a “boutique state” for Assad.

But Russia countered saying it is only targeting groups it considers to be “terrorists”, and accused Turkey of preparing an invasion in northern Syria.

Nato has criticised Russia for 'undermining' efforts to negotiate a peaceful end to the war (AFP)

250,000 people are estimated to have been killed in the fighting over the past five years with 11m people displaced.

Turkey has taken in more than 2.5m million refugees - more than any other country in terms of the total number.

Additional reporting by Reuters