Beirut: The United Nations urged Turkey to let in tens of thousands of Syrians fleeing a regime offensive around Aleppo on Tuesday, adding to calls for Russia to end air strikes ahead of fresh peace efforts.

Up to 31,000 people have fled Aleppo and surrounding areas since last week, as government forces backed by Russian warplanes press an offensive that threatens to encircle the rebel-held eastern part of Syria’s second city.

‘The highest need and the best humanitarian response is for the bombing to stop,’ UN aid chief Stephen O’Brien said, when asked if Russia should halt its air campaign in Aleppo. ‘All bombings should stop.’

UN refugee agency (UNHCR) spokesman William Spindler urged Turkey to open its border to ‘all civilians from Syria who are fleeing danger and seeking international protection’.

Huge crowds of Syrians, most of them women and children, have spent days waiting at the Oncupinar border crossing into Turkey, sleeping in the open or packed into tents.

Ahmad al-Mohammad, a field worker with medical aid group Doctors Without Borders, known by its French initials MSF, said crowded conditions were causing health problems including diarrhoea.

‘There are no longer enough places for families to sleep,’ said told AFP. ‘Most of the families left with just the clothes they were in.’

Turkey, which already hosts 2.5 million Syrians, is delivering supplies across the border but has said it will let the new arrivals in only ‘if necessary’.

Deputy Prime Minister Numan Kurtulmus has said that a ‘worst case scenario’ could see up to 600,000 refugees arrive at the border.

‘Our objective for now is to keep this wave of migrants on the other side of Turkey’s borders as much as is possible, and to provide them with the necessary services there,’ Kurtulmus said.

Focus on Munich talks

The Aleppo offensive is piling on the pressure for a political solution ahead of a 17-nation contact group meeting Thursday in Munich aimed at getting peace talks back on track.

US Secretary of State John Kerry warned that Russia’s aerial bombardment of Syrian opposition targets could derail efforts to revive the peace process, after discussions collapsed last week.

‘Russia’s activities in Aleppo and in the region right now are making it much more difficult to be able to come to the table and to be able to have a serious conversation,’ Kerry said in Washington.

‘We have called on Russia — and we call on Russia again — to join in the effort to bring about an immediate ceasefire.’

EU president Donald Tusk said the Russian air strikes were ‘making an already very bad situation even worse’.

‘As a direct consequence of the Russian military campaign, the murderous Assad regime is gaining ground, the moderate Syrian opposition is losing ground and thousands more refugees are fleeing towards Turkey and Europe.’

NATO said it would take any request to help with the refugee crisis ‘very seriously’, after Ankara and Germany said they would seek the alliance’s help combating people smugglers.

US Defence Secretary Ashton Carter is also expected to discuss the situation in Aleppo during a trip to Europe this week designed to drum up support for the fight against Islamic State jihadists in Syria and Iraq.

Under siege

Syria’s nearly five-year-old conflict has claimed 260,000 lives and displaced half the population.

The UN has warned 300,000 people in eastern Aleppo city could be cut off from humanitarian aid if government forces encircle the area, a tactic used by the regime to devastating effect against other former rebel bastions.

A report from Washington-based The Syria Institute and PAX, a peace organisation based in the Netherlands, said Tuesday that more than one million Syrians are living under siege.

The UN’s World Food Programme said it had begun food distributions to the displaced, despite the severing of access and supply routes.

‘We are making every effort to get enough food in place for all those in need,’ said WFP Syria country director Jakob Kern.

Syrian government forces, backed by Russian air raids, began a major operation in the northern province of Aleppo last week and are now around 20 kilometres (12 miles) from the Turkish frontier.

The regime advances came as peace talks in Geneva collapsed last week in part over rebel anger about the government offensive.

More than 20 suspected Russian air strikes hit several towns northwest of Aleppo city and in the northern countryside on Tuesday, said the Observatory, which relies on a network of sources on the ground.