Thousands of migrants and refugees massed at Turkey's western frontier on Sunday, trying to enter Greece by land and sea after Turkey said its borders were open to those hoping to head to Europe. In Syria, Turkish troops shot down two Syrian warplanes after the Syrian military downed a Turkish drone, a major escalation in the direct conflict between Syrian and Turkish forces.

Turkey's decision to ease border restrictions came amid a Russia-backed Syrian government offensive into Syria's northwestern Idlib province. That offensive has killed dozens of Turkish troops and led to a surge of nearly a million Syrian civilians fleeing the fighting toward Turkey's sealed border.

Turkey backs the Syrian rebels fighting in Idlib province, and has sent thousands of troops into the area. Idlib is the last opposition-held stronghold in Syria, and is dominated by al-Qaida linked fighters.

A Turkish official said the fighting in Idlib was directly linked to Turkey's decision to open the gates for refugees to Europe. He said Ankara had changed its focus to preparing for the possibility of new arrivals from Syria “instead of preventing refugees who intend to migrate to Europe.”

“Europe and others must take robust action to address this monumental challenge,” said Fahrettin Altun, the communications director for Mr. Erdogan. “We can’t be expected to do this on our own.”

Mr. Erdogan's decision to open his country’s borders with Europe made good on a longstanding threat to let refugees into the continent. His announcement marked a dramatic departure from a previous policy of containment, an apparent attempt to pressure Europe into offering Turkey more support in dealing with the fallout from the Syrian war to its south.

Under a 6 billion euro deal in 2016, Turkey agreed to stem the tide of refugees to Europe in return for financial aid, after more than a million people entered Europe in 2015. Turkey has since accused the EU of failing to honor the agreement, and Mr. Erdogan has frequently threatened to allow refugees into Europe unless more international support was provided.

Turkey already hosts 3.6 million Syrian refugees, as well as many others from Africa, Asia, and the Middle East. Turkey borders Greece and Bulgaria, both European Union members.

On the Greek-Turkish land border, Greek army and police patrols using tear gas and stun grenades to thwart attempts by thousands to push into the country overnight.

Officials said the situation was much calmer Sunday morning. But in the afternoon, authorities used tear gas and water cannons to push back another crowd attempting to cross. Migrants threw rocks and other objects, and one policeman was injured. Greek authorities said they stopped about 10,000 crossing attempts Saturday, and another 5,500 on Sunday.

Greek Prime Minister Kyriakos Mitsotakis convened the defense and foreign affairs committee Sunday evening. Afterward, a government spokesman said Greece was starting a one-month freeze on accepting asylum applications from migrants who enter illegally.

Europe's border agency Frontex said it was “redeploying equipment and additional officers to Greece.”

A Greek government official said the Turkish authorities also fired teargas at the Greek border, using drones flying close to the border. The official spoke on condition of anonymity because he wasn't authorized to discuss the matter with the media.

Stavros Zamalides, the president of the Greek border community of Kastanies, said Turkish soldiers used wire cutters to actively help people cross.

The United Nations migration organization reported at least 13,000 people had massed on Turkey’s land border by Saturday night, the vast majority apparently from Afghanistan.

In Istanbul, a steady stream of buses, taxis, cars and minibuses were ferrying hundreds more throughout Sunday to Edirne, a town near the border with Greece. The vehicles weren’t part of any regular bus route.

Those boarding the buses – the vast majority Afghans – said they were heading to Greece and eventually hoped to get to Germany.

On the Greek islands, more than 500 people had arrived from the nearby Turkish coast by Sunday evening, a clear increase in the usual number of people who arrive on eastern Aegean islands from Turkey.

Existing migrant camps on the islands are already dramatically overcrowded, and tensions there have mounted.

In a small harbor on Lesbos, angry local residents refused to allow migrants – including families with young children and babies – to disembark from a dinghy that had just arrived. Groups who arrived on other parts of the island remained there for hours because locals prevented buses from reaching them to transport them to the main camp.

On Sunday night, a former staging area used for new arrivals on Lesbos was set on fire.

More than 19,300 people already live in and around island’s migrant camp, which has a capacity of 2,840. Protests by island residents last week over the situation degenerated into clashes with riot police on Lesbos and Chios.

Greece said it was using “all available means” to tell migrants that the country’s borders were closed, including text messages to foreign mobile phones in the border area.

In Syria, fighting escalated on the ground and in the skies between Turkish and Syrian troops. Russia, a key ally of Syrian President Bashar Assad's government, remained largely on the sidelines through the weekend even as a Turkish-led counter offensive blunted and in some cases reversed Syrian government advances.

But the head of the Russian military’s Reconciliation Center in Syria, Rear Adm. Oleg Zhuravlev,, warned Turkey that its aircraft could be in danger if they fly over Syria.

“In view of the sharp exacerbation of tensions in the air space over Idlib, the Syrian government was forced to declare it closed,” Mr. Zhuravlev said in a statement released late Sunday. “In this situation, the Russian troops’ command can’t guarantee security of flights of Turkish aircraft in the skies over Syria.”

Syria’s official news agency SANA said the four pilots in the two jets shot down Sunday had ejected and landed safely. Turkey's Defense Ministry said it had hit the two SU-24 aircraft as well as Syrian air defense systems after one of its aerial drones was downed.

Earlier Sunday, Syria said northwestern airspace was closed and any aircraft or drone that entered “will be treated as hostile and shot down.”

The Syrian announcement followed two days of Turkish drone strikes in Idlib province. Syrian activists said the strikes killed more than 50 Syrian government forces and allied fighters. Turkey has lost 54 soldiers in February, including 33 killed Thursday in a single airstrike. Outraged, Mr. Erdogan announced his country's European borders were open Saturday.

The crisis in Idlib stems from a Syrian government offensive with Russian military support, which began Dec. 1. Turkey is worried it might come under renewed international pressure to open its now-sealed border with Syria and offer refuge to hundreds of thousands more Syrian civilians.

Turkish Defense Minister Hulusi Akar, speaking from a military headquarters near the Syrian border, said Turkey aimed to confront Syrian government forces rather than Russian troops. He called on Moscow to persuade Assad to withdraw to 2018 cease-fire lines on the edges of Idlib.

Referring to losses inflicted on Syria, he said Turkey had “neutralized” more than 2,200 Syrian troops, 103 tanks and eight helicopters. The operation is Turkey's fourth in the war-torn country since 2016.

Mr. Altun, the Turkish communications director, claimed 80,888 migrants had left Turkey for Europe “in the past several days."

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There was no evidence to support his claim. Greece’s Foreign Ministry tweeted that these numbers were “false and misleading."

This story was reported by The Associated Press.