UN agencies on Tuesday called on Turkey to let in masses of refugees fleeing a government offensive in northern Syria and warned that up to 300,000 people in rebel-held eastern Aleppo city faced being cut off from aid.

Local activists and a monitoring group meanwhile said intense Russian airstrikes in support of government forces were continuing in the countryside between Aleppo and the Turkish border.

The Turkish government has so far kept the border closed to the more than 30,000 people stranded in the area after Syrian government forces cut off routes to Aleppo and central Syria.

"We are asking Turkey to open its borders to all refugees fleeing Syria," UN refugee agency spokesman William Spindler said.

Turkey's generosity in taking in Syrians so far, and its aid efforts inside Syria, were "not replacing the obligation to take in people fleeing from danger," he added.

Turkey did modify its policy Tuesday and began to allow injured refugees to enter via the Bab al-Salameh crossing, a spokesman for the aid organization IHH told dpa by telephone. However, it remains closed to all others.

The wounded are being treated in Turkish hospitals, says Mustafa Ozbek of the IHH, which is close to the Turkish government.

Deputy Prime Minister Numan Kurtulmus told state-run Anadolu news agency on Monday that as many as 600,000 new refugees could make their way to the border "in a worst case scenario."

Some 80 per cent of those fleeing were women and children, the UN's Office for the Coordination of Humanitarian Affairs said.

The office warned that up to 300,000 people could be cut off from cross-border supplies as government forces moved south to capture the remaining route between rebel-held eastern Aleppo city and Turkey.

Other sources have put the number of civilians still inside eastern Aleppo, which has been devastated by government shelling and airstrikes, much lower.

The UN World Food Programme said it had started distributing aid to displaced peopole in the rebel-held enclave in northern Aleppo.

Over the next few days parcels containing a month's supply of food would be distributed to a further 21,000 people, the agency said.

The Syrian Observatory for Human Rights and local activists reported continuing heavy airstrikes, thought to be Russian, in the enclave throughout Tuesday.

Ammar Jello, an activist based near the border, said that Tel Rifaat, the nearest town to advancing government forces, had seen 30 to 35 airstrikes during the day.

"We miss the Syrian airstrikes," another opposition activist inside Tel Rifaat told dpa, saying those now being carried out by the Russians were more intense.

Medical charity Medecins sans Frontieres (MSF) meanwhile said that three people had been killed in an airstrike on a hospital supported by the group in southern Syria.

Six others were injured in the strike on a field hospital in Tafas, 12 kilometres from the Jordanian border, on Friday night, the charity said, and 20,000 people fled the town for nearby countryside.

MSF did not assign responsibility for the airstrike. The Syrian government and allied Russian forces have been carrying out intense strikes in the area.

Meanwhile, in Damascus, the Observatory reported that eight policemen were reported killed in a suicide attack on an officers' club.

State news agency SANA said the blast, which was claimed by the Islamic State extremist group, hit a vegetable market, killing three people and injuring 14.

Also on Tuesday, Turkey summoned the US ambassador to protest a statement by US State Department spokesman John Kirby that his country did not regard powerful Syrian Kurdish forces as terrorists, Anadolu reported.

Kirby had been asked to comment on remarks by Turkish President Recep Tayyip Erdogan, who on Sunday lashed out over a visit to the Syrian Kurdish town of Kobane by a senior US official.

Kirby on Tuesday said nothing had changed about US policy toward the Democratic Union Party (PYD), which dominates the three Kurdish autonomous regions in northern Syria, but reiterated that the United States considers the Kurdistan Workers' Party (PKK) a foreign terrorist organization.

Syria's Kurdish forces are key US allies on the ground against Islamic State, but Turkey has repeatedly condemned them as terrorists due to their close links with Kurdish rebels fighting on its soil.

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