MOSCOW >> After a week of swirling controversy, Russia acknowledged today that five of its citizens may have been killed by a U.S. strike in Syria, the first time Russians have died at U.S. hands in Syria.

Russian Foreign Ministry spokeswoman Maria Zakharova emphasized that the five victims weren’t Russian servicemen.

“According to preliminary information, five people, presumably Russian citizens, may have been killed in combat, the circumstances of which are being clarified,” Zakharova said in a briefing. “Some others have been injured, and it’s still necessary to check whether they are citizens of Russia or other nations.”

Until Thursday, both Russian and U.S. officials said they had no information on Russian casualties in the Feb. 7 clash, which came when pro-Syrian government forces attacked positions of the U.S.-backed Syrian Kurdish fighters in the oil-rich eastern province of Deir el-Zour and faced a ferocious U.S. counterattack.

Zakharova wouldn’t describe how the Russians died, saying the circumstances of the clash are still being clarified, but her statement amounted to official recognition that the victims were part of the force that advanced on positions of the U.S.-backed fighters.

It marks the first time Russian and U.S. combatants have become directly engaged in combat in Syria — a scenario Moscow and Washington have anxiously sought to avoid.

On the chaotic Syrian battlefield, Russian forces are supporting the Syrian government in the fight against opposition groups, some of which are backed by the United States, and elements of both sides are fighting the last remnants of the Islamic State group in Syria.

The U.S. military has said it maintained contact with the Russian military in Syria before, during and after the Feb. 7 clash. The Russian Defense Ministry insisted its troops weren’t involved in the incident, saying 25 Syrian volunteers were wounded in the U.S. strike.

But Russian news media and social networks swirled with reports of the combat, describing how U.S. aircraft decimated Russian private military contractors who sought to take control over an oil factory near Khusham. Some reports put Russian losses at 200 or more, and a growing chorus of politicians, commentators and bloggers slammed the Kremlin for failing to acknowledge the casualties.

President Vladimir Putin’s spokesman, Dmitry Peskov, has said that some Russian citizens could be in Syria but the Kremlin doesn’t have any information about them. Pressed by reporters in Thursday’s conference call whether Putin ordered information about Russian casualties kept from the public, Peskov denied that.

Along with the Russian military, which has waged a military campaign in support of Syrian President Bashar Assad’s government since September 2015, thousands of Russians have also reportedly fought there as private contractors. The private fighters allow the Kremlin to keep the official death toll from its campaign in Syria low, helping to avoid negative publicity about Russia’s involvement in Syria as Putin runs for re-election in the country’s March 18 presidential vote.

The push for oil assets appears to have been the top mission for Russian private contractors in Syria.

The U.S.-backed Kurdish-led forces have been vying with Russian-backed Syrian troops reinforced by Iranian-supported militias for control of the oil-rich Deir el-Zour province. It was unclear why pro-government forces acted so brazenly on Feb. 7 despite facing the obvious risk of a punishing U.S. counterstrike.

Observers noted the Russian military might have lacked information about the private contractors’ move on oil facilities because of poor coordination between them.

In the Feb. 7 clash, the U.S. military said a battalion-sized formation backed by tanks and artillery attacked the U.S.-backed fighters and the U.S. responded by unleashing a broad range of air power. For more than three hours, American F-15E attack jets, B-52 strategic bombers, AC-130 gunships, Apache attack helicopters and Reaper drones fired on the attacking force, killing about 100 attackers and destroying an unspecified number of artillery guns and battle tanks, said Lt. Gen. Jeffrey Harrigian, commander of U.S. air forces in the Middle East.

Some said the offensive was launched because of a rumored relocation of some of the fighters from the area to the Kurdish enclave of Afrin in northwestern Syria, which is facing a Turkish offensive.

Turkish Defense Minister Nurettin Canikli said Thursday he asked U.S. Secretary of Defense Jim Mattis for the United States to end its support for Syrian Kurdish fighters and remove them from a U.S.-backed Syrian Democratic Forces, or SDF, that is fighting the Islamic State group in Syria.

Canikli said in comments televised live from Brussels that he told Mattis that U.S. support for the Syrian Kurdish militia, known as YPG, has helped Kurdish rebels in Turkey “to grow and strengthen,” posing an increasingly “existential” threat to Turkey.

Canikli said he presented documents to Mattis proving “organic” links between the YPG and Kurdish rebels in Turkey.

U.S. Secretary of State Rex Tillerson is due in Ankara later on Thursday to discuss growing tensions between the two NATO allies.

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Suzan Fraser in Ankara contributed to this report.