Thousands of migrants have massed at the Greek frontier with Turkey since President Recep Tayyip Erdogan announced last week that they would no longer be prevented from trying to enter Europe.

Erdogan's move came after 34 Turkish troops were killed in northern Syria by Russian-backed Syrian forces, prompting him to seek greater assistance from the international community.

But EU leaders now fear a repeat of the migrant crisis of 2015-16, when more than one million migrants crossed into the EU, and have decried Turkish "blackmail".

With mounting tensions around the border crossing at Pazarkule, a Turkish official claimed one migrant was killed and five injured by live fire from the Greek side.

Athens strongly denied the claim, but an AFP photographer earlier saw a migrant shot in the leg -- it was not clear whether by a real or rubber bullet -- as a group of refugees tried to cut their way through fencing.

The group then threw stones at the Greek police, who responded with tear gas, while multiple shots and cries were heard.

Athens released a video apparently showing Turkish police firing tear gas at Greek border guards, which could not be immediately confirmed.

Speaking in Ankara, Erdogan said Europe must support Turkey's "political and humanitarian solutions in Syria" if it wants to resolve the situation.

Turkey already hosts nearly four million refugees, most of them Syrians, and has been fighting the Syrian government in a bid to prevent another influx from Idlib, the jihadist-dominated region that has been under attack by Damascus since December.

Close to one million people in Idlib have been displaced by the government assault, which is backed by Russian air power, though they are currently blocked from entering Turkey.

EU foreign policy chief Josep Borrell and Council President Charles Michel met with Erdogan and other top officials in Ankara on Wednesday, promising an additional 170 million euros ($189 million) in aid for vulnerable groups in Syria.

Borrell said the EU recognised the "difficult situation Turkey is facing" but that Turkey's decision to open the way for migrants could "only make the situation worse".

- Ceasefire? -

Despite being on opposing sides of the nine-year war in Syria, NATO-member Turkey and Russia have kept lines of communication open.

But the relationship has been heavily strained as more than 50 Turkish soldiers have died in Idlib in recent weeks.

Erdogan said he hoped a ceasefire would be "swiftly established" when he meets his counterpart Vladimir Putin in Moscow on Thursday.

Turkey's defence ministry said Wednesday that three more soldiers had been killed by government fire in the past 24 hours.

Turkey said it had "immediately" retaliated, and the Syrian Observatory for Human Rights said nine government troops were killed by Turkish drone strikes in the Saraqeb area.

Ankara officially announced an offensive against Syrian forces over the weekend, demanding they pull back behind lines agreed under the 2018 Sochi deal with Russia.

"We expect Russia to fulfil its promises as a guarantor country and stop the regime's attacks and to use its influence to ensure the regime adheres to the Sochi deal's borders," Turkish Defence Minister Hulusi Akar said, according to broadcaster NTV.

But many say Russia is determined to see the Syrian government regain full control of its territory.

"There might be a ceasefire announced after the talks between Putin and Erdogan but it'll be for show," a Western diplomat told AFP.

"I believe Putin will tell Erdogan that's it for his actions in Syria."

- Support Turkey -

The EU has scrambled to respond to the surge of migrants at the Greek border, where authorities say some 24,000 were stopped from entering between Saturday and Monday.

Turkey agreed in 2016 to stop the flow of refugees in exchange for billions of euros, but says the EU failed to honour other parts of the deal, such as visa liberalisation and an improved customs agreement.

Erdogan criticised the Greek response, saying: "The Greeks -- who are resorting to any means to stop refugees coming into their country, even drowning them or killing them with live ammunition -- they shouldn't forget they might need this same mercy one day."

A Syrian refugee near the border in Turkey's Edirne province told AFP the Greek authorities were shooting at them.

"Whoever goes is immediately hit, they threw tear gas," he said.

Meanwhile, Greece sent a navy ship to the island of Lesbos to house some 500 people, many of them families with small children, who managed to reach its shores over the weekend.