Greek police fired tear gas and used water cannons to stop Syrians trying to cross land borders from Turkey on Saturday, according to AFP and Reuters reporters at the scene.

Clashes erupted between police and hundreds of Syrians, who responded by hurling rocks and firing projectiles towards the Greek border town of Kastanies.

More than 140,000 asylum seekers have left for Greece since Turkey's sudden decision to open its borders following the killing of at least 33 soldiers in the war-torn Syrian province of Idlib last month.

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Hundreds of thousands of people are fleeing Syrian and Russian bombardment in the province.

Meanwhile, Turkish President Recep Tayyip Erdogan has ordered Turkey's coastguard to prevent migrants crossing the Aegean sea because of the risks, officials said.

"On the orders of the president... permission will not be given for migrants to cross the Aegean sea because it is dangerous," the coastguard tweeted on Friday.

"The approach of not intervening against migrants wishing to leave Turkey remains in practice but this [new] approach covers sea crossings because of the dangers," it added in another tweet.

The coastguard said 97 migrants were rescued on Thursday after "the Greek side flattened three boats and left them in a half-sinking state in the middle of the sea".

Also, a fire engulfed a refugee shelter on the Greek island of Lesbos on Saturday. The fire at One Happy Family, a Swiss-operated family care centre for refugees just outside the island capital, came after there was violence at the weekend directed at aid groups and journalists on Lesbos.

"The school building has a lot of damage, we can't say more at the moment," a source among the operators told AFP. "The fire brigade is there, our team on the ground as well," they added. There were no immediate reports of injuries.

'They are being used by Turkey'

Greek officials say authorities have thwarted thousands of attempts by refugees and migrants to cross in the past eight days.

Over a 24-hour period by Saturday morning, there were more than 1,200 attempts to cross and 27 arrests, a government source told Reuters.

Most of the migrants were from Afghanistan and Pakistan.

"Greece is doing what every sovereign state has the right to do, to protect its border from any illegal crossings," Greek Prime Minister Kyriakos Mitsotakis told the CNN network on Friday night.

"I'm afraid this is a constant and very systematic provocation on Turkey's behalf which has nothing to do with the plight of these people. They are being used by Turkey."

Heavy machinery was moved to the border crossing area on the Greek side on Saturday morning, Reuters said, and a bulldozer was seen digging embankments close to Kastanies.

EU-Turkey tensions

The EU on Friday accused Turkey of creating a crisis by opening its borders to refugees and migrants wanting to head to Europe.

More than 140,000 asylum seekers have left for Greece, Turkish interior minister Suleyman Soylu tweeted on Friday.

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Turkey announced last week that it would no longer stop migration as agreed in a 2016 deal with the EU. The refugees and migrants have been forcefully blocked by Greek authorities.

Turkey has denied that it is using the refugees for political purposes. Foreign Minister Mevlut Cavusoglu told German newspaper Bild that Turkey "cannot shoulder this heavy burden on its own".

At least two refugees have reportedly died in the past week - a four-year-old child, who drowned when a boat capsized, and a man shot dead by Greek forces, according to Turkish security sources.

Although Greece denounced the man's death as "fake news", investigative group Forensic Architecture said analysis of open-source information suggested video footage of the shooting was real.