BEIRUT (AP) — Turkey sent more reinforcements into northwestern Syria on Thursday, setting up new positions in an attempt to stop a Syrian government offensive on the last rebel stronghold in the war-torn country, state media and opposition activists said.

The move came after a rare confrontation between Turkey and Syria on Monday that killed seven Turkish soldiers and a Turkish civilian member of the military, as well as 13 Syrian soldiers.

In Moscow, the Foreign Ministry announced that Russian soldiers have been killed in a northern Syrian province alongside Turkish servicemen, without saying when the incident occurred. The ministry blamed “terrorists” for the deaths, saying that the attacks in Idlib province intensified in January.

“Russian and Turkish military specialists died tragically,” the statement said, without specifying when and how many Russian soldiers were killed.

Moscow and Ankara are on opposite sides of the Syrian civil war — Russia is a main backer of Syrian President Bashar Assad's forces while Turkey supports insurgents fighting his government.

Syrian troops have been advancing since December into the country's last rebel stronghold, which spans across the Idlib province and parts of nearby Aleppo region. Turkish troops are deployed in some of those rebel-held areas to monitor an earlier cease-fire that has since collapsed.

Turkey has set up four military posts in northwestern Syria to prevent Syrian government forces from marching deeper into Idlib, Syria's Foreign Ministry said,. It added that Turkish troops have “flagrantly violated” Syria's border and deployed in several areas, including the villages of Binnish, Taftanaz and Maaret Musreen.

The offensive has led to a humanitarian crisis with more than 520,000 fleeing their homes, according to the U.N. Many of them have sought safety in areas close to the border with Turkey since the beginning of December. At least 53 health facilities have suspended work in the area, according to the World Health Organization.

The European Union called Thursday for an end to the fighting around Idlib and urged warring factions to allow aid workers and supplies into the area.

"Bombings and other attacks on civilians in north-west Syria must stop," EU foreign policy chief Josep Borrell and the bloc's Crisis Management Commissioner Janez Lenarcic said in a statement.

They said the EU, a major aid donor, would continue to provide help but that “unimpeded, safe and secure access” was required.

The Britain-based Syrian Observatory for Human Rights, an opposition war monitoring group, said Syrian warplanes on Thursday attacked a military air base in the village of Taftanaz where Turkish troops deployed recently.

The Observatory and Syrian state TV said that government forces have laid a siege on the town of Saraqeb, which sits on the intersection of two major highways, one linking the Syrian capital of Damascus to the north, and the other connecting the country’a west and east.

The Observatory said Syrian rebels launched an attack later Thursday under the cover of intense Turkish artillery shelling on the village of Nairab, which Syrian troops captured earlier this week.

During the attack, insurgents carried out at least one suicide attack with an armored personnel carrier rigged with explosives, according to citizen journalist Taher al-Omar, who is embedded with the militants on the edge of the village.

Turkish leaders have repeatedly called on Russia to “rein in” Syrian government forces, a demand to which Moscow responded by expressing concerns over growing “terrorist” activity in Idlib.

Kremlin spokesman Dmitry Peskov reiterated the notion on Thursday, saying that both Russia and Turkey each have “their own sets of concerns." The concentration of insurgent groups in Idlib and their “continuing activity” was Russia's main issue, he said.

Peskov refused to say how many Russians were killed in Idlib, but said the Kremlin doesn't rule out a meeting between Russian President Vladimir Putin and his Turkish counterpart, Recep Tayyip Erdogan, to discuss the situation in Syria.

Meanwhile, Turkish Foreign Minister Mevlut Cavusoglu said a delegation from Russia would arrive in Turkey soon for talks on the situation in Idlib following the attack on Turkish soldiers. He said a follow-up meeting between Erdogan and Putin could also take place "if there is a need."

Speaking to Turkish reporters during a visit to Azerbaijan, Cavusoglu also renewed a call for Russia to stop "increased" Syrian attacks in Idlib.

Idlib province is home to some 3 million people, many of them displaced from other parts of Syria in earlier bouts of violence.

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Litvinova reported from Moscow. Associated Press writer Suzan Fraser in Ankara, Turkey, contributed to this report.