President Recep Tayyip Erdogan said his government had begun to allow refugees to travel on to Europe from Turkey, which he said can no longer handle new waves of people fleeing war-torn Syria.

"What did we do yesterday? We opened the doors," Erdogan told the Turkish Parliament on Saturday, in his first comments since 33 Turkish troops were killed in northern Syria on Thursday.

"We will not close those doors ... Why? Because the European Union should keep its promises."

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In 2016, Turkey signed a deal with the EU to stop refugees crossing from its borders after the 2015 migrant crisis that saw one million crossed the Aegean into Europe.

Erdogan also said 18,000 refugees had gathered on the Turkish borders with Europe since Friday, adding that the number could reach as many as 30,000 on Saturday.

Turkey, which is already home to the world's largest number of refugees - around 3.6 million Syrians, has repeatedly warned that is is overburdened.

"We are not in a situation to handle a new wave of refugees" from Syria, Erdogan said.

'Tense situation'

Erdogan has previously threatened to "open the gates" unless more international support was provided, particularly at times of tension with European countries.

Al Jazeera's Natasha Ghoneim, reporting from Istanbul, said: "We are waiting to find out if this is a temporary measure to send a message to NATO and Europe or if this may be something more."

The killing of Turkish troops has increased tensions with Russia, which backs the Syrian regime's offensive to take back the remaining chunks of the northwestern Idlib region. Turkey backs Syrian rebels.

Meanwhile, thousands of refugees stuck on the Turkey-Greece border clashed with Greek police on Saturday.

Greek riot police fired tear gas at the refugees at a border crossing in the western Turkish province of Edirne, some of whom responded by hurling stones.

"What you have right now is a tense situation," said Al Jazeera's Ghoneim, adding that women and children were among the refugees.

"They are in a kind of no man's land, trapped between the official borders of Turkey and Greece," she said.

On Saturday, small groups managed to get across into Greece without documentation.

Most were from Afghanistan, including some families with young children.