At least 15,000 Syrian refugees fleeing fighting in northern Aleppo province have gathered at a border crossing with Turkey, UN and Turkish officials said.

The frontier is shut but Turkey has said it is prepared to feed and shelter the refugees.

In the past few days, the Syrian army backed by Russian air power has made a series of gains in Aleppo province.

The advance threatens to surround the city of Aleppo, Syria's largest, landing a major blow to the rebels.

Nato has accused Russia of "undermining" Syrian peace efforts through its strikes, which it says are mainly aimed at opposition groups. Russia insists it only targets what it calls terrorists.

US Secretary of State John Kerry said talks were under way about a ceasefire to provide humanitarian access in Syria and the next few days would determine if "people are serious, or people are not serious".

Turkish President Recep Tayyip Erdogan accused Russia of being engaged in an "invasion" of Syria, saying it was trying to create a "boutique state" for ally President Bashar al-Assad.

Mr Erdogan said Russia and the Syrian government were together responsible for 400,000 deaths in Syria.

On Thursday Russia accused Turkey, key backer of Syria's opposition, of preparing a ground invasion, an accusation Mr Erdogan called "laughable".

Turkey and Russia have been embroiled in a row since Turkey shot down a Russian jet it accused of violating its airspace in November.

According to the UN, up to 20,000 Syrians have gathered at the Bab al-Salam border crossing.

"Humanitarian organisations are responding to the needs of those displaced, but ongoing military conflict is making access to populations in need increasingly difficult," the UN's Linda Tom told the AFP news agency.

She said another 5,000-10,000 displaced people had gathered at Azaz, a north-western Syrian city close to the Turkish frontier.