BEIRUT/LONDON - The United Nations on Tuesday urged Turkey to open its borders to tens of thousands of Syrians who have overwhelmed nearby emergency camps after fleeing a major government offensive.

The main border crossing north of Syria’s second city Aleppo remained closed, forcing huge crowds including women and children to sleep in tents or in the open.

“There are no longer enough places for families to sleep,” said Ahmad al-Mohammad, a field worker with medical aid group Doctors Without Borders (MSF). “Most of the families left with just the clothes they were in,” he told AFP, adding that the cold and the crowded conditions were causing health problems including diarrhoea.

The United Nations says up to 31,000 people have fled Aleppo city and surrounding areas in recent days, as government forces backed by Russian warplanes press an offensive that could encircle the rebel-held part of the city.

“We are asking Turkey to open its border to all civilians from Syria who are fleeing danger and seeking international protection,” said UN refugee agency (UNHCR) spokesman William Spindler.

EU president Donald Tusk said 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.” Syria’s nearly five-year-old conflict has claimed 260,000 lives and displaced half the population.

On Tuesday a suicide car bomb claimed by the Islamic State group killed at least nine people at a police club in the north of Damascus, according to the Syrian Observatory for Human Rights, a Britain-based monitor.

Turkey’s Deputy Prime Minister Numan Kurtulmus said Monday 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. Turkey already hosts 2.5 million Syrian refugees, but has come under pressure to allow in more, as well as to prevent them from seeking to reach Europe. NATO said Tuesday 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 combatting people smugglers.

Turkey’s Oncupinar border crossing north of Aleppo city remained closed Tuesday with only medical emergencies allowed through. A Turkish official said four wounded Syrians had been let in on Monday. The UN’s humanitarian aid agency OCHA said on Monday that eight informal camps on the Syrian side of the border were at “full capacity”. MSF said aid groups were distributing warm clothes and mattresses to those stranded on the Syrian side. “They are trapped,” Mohammad said. “They’ve left their homes and everything they have behind, and they can’t get into Turkey.” The UN warned that 300,000 people in eastern Aleppo city could be cut off from humanitarian aid if government forces encircle the area.

Government sieges have been employed 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. It said the crisis was “far worse” than acknowledged by the UN, which in January estimated the number at 486,700. The World Food Programme said it had begun food distributions to the displaced, despite the severing of access and supply routes in the region. “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 allied militias and Russian air strikes began a major operation in the northern province of Aleppo last week. They are now around 20 kilometres 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 21 suspected Russian air strikes hit targets in several towns northwest of Aleppo city and in the northern countryside on Tuesday, the Observatory said. Suspected Russian air strikes also hit parts of Tal Rifaat during the night, according to the monitor, which relies on a network of sources on the ground.

Regime forces and their allies meanwhile were fighting rebels, including miitants from Al-Qaeda-linked Al-Nusra Front, in the southwestern countryside of Aleppo province.

Meanwhile, ten times as many migrants and refugees arrived in Europe by sea in the first six weeks of the year as in the same period of 2015, and the number of deaths also soared, the International Organization for Migration (IOM) said on Tuesday.

The number of arrivals topped 76,000, and the number of deaths shot up to 409 on Mediterranean routes from 69 in the first six weeks of 2015, it said.

The IOM also said it expected no fall in the number of arrivals in Europe and predicted that next month Greece would receive its one millionth arrival since the migrant crisis began.

More than 1.1 million people fleeing poverty, war and repression in the Middle East, Asia and Africa reached Europe’s shores last year, most of them heading for Germany.

Around half the arrivals are refugees from the Syrian war, the UN Refugee Agency (UNHCR) says.

Meanwhile, Swedish police said Tuesday they had arrested 14 men for allegedly planning to attack an asylum centre after finding axes, knives and iron pipes in their cars.

Authorities said they detained the men on Monday after being tipped off about a planned attack against a migrant centre in Nynashamn, some 60 kilometres south of Stockholm.

The suspects were all carrying foreign ID papers, the daily Aftonbladet quoted police as saying, adding some of them were Polish nationals living and working in Sweden. “We believe that the migrant centre was the target of the attack,” police spokesman Hesam Akbari told AFP. He would not however be drawn on the suspects’ nationalities.

Last month, dozens of masked men believed to belong to hooligan or neo-Nazi gangs gathered in central Stockholm and distributed leaflets calling for attacks on young migrants.

Sweden, a country of 9.8 million, took in about 163,000 asylum seekers in 2015, putting it among the European Union states with the highest proportion of refugees per capita.

More than one million people travelled to Europe last year - the majority of them refugees fleeing conflict in Syria, Iraq and Afghanistan - triggering the continent’s worst migration crisis since World War II. With the influx showing little sign of abating, many countries, including Austria, Germany, Denmark, Sweden, France, have tightened their asylum rules in a bid to discourage new arrivals.

In Sweden, the tougher rules comes against the backdrop of rising concern over conditions in the country’s overcrowded asylum facilities.

Swedish officials have called for stepped-up security after an employee at a centre for unaccompanied child refugees was fatally stabbed in January.

Sweden expects to receive up to 140,000 additional asylum seekers this year.