During the war in Syria, Russia and the United States have maintained a communications channel to avoid collisions between their warplanes and soldiers in the chaotic battlefield. If Russian deaths from the U.S. strike are confirmed, it would mean the two countries were on the verge of a direct military confrontation in Syria.

The episode could further strain ties between the two countries, which have plummeted to the lowest levels since the Cold War over the Ukrainian conflict, the war in Syria and the alleged Russian meddling in the 2016 U.S. presidential election.

The U.S. military said last week it unleashed air and artillery barrage on Syrian government-backed troops after some 300 attackers launched what appeared to be a coordinated assault on Syrian opposition forces accompanied by U.S. advisers in the province of Deir el-Zour. It said it was in regular communication with Russian counterparts before, during and after the attack, and Russian officials assured coalition officials they would not strike coalition forces.

It said the U.S. strikes, which reportedly killed about 100 of the attackers, were in self-defense.

The Russian Defense Ministry has insisted that its troops weren’t involved in the incident, saying 25 Syrian volunteers were wounded in the U.S. strike.

It also noted pro-government forces that advanced toward the area under control of the U.S.-supported forces had failed to coordinate their action with the Russian military in Syria.

Mattis last week dismissed any suggestion that Russia had any control over the attacking force, whose nationalities, motives and makeup he could not identify.

The U.S. strike came as U.S.-backed Kurdish-led forces were vying with Russian-backed Syrian troops that are reinforced by Iranian-supported militias for control of the oil-rich Deir el-Zour province.

The Russian Defense Ministry charged that the Feb. 7 incident reflected a U.S. push to grab Syria’s economic assets under the cover of fighting the Islamic State group.

Without mentioning the U.S. strike, Russian Foreign Minister Sergey Lavrov noted Tuesday that “Americans have taken dangerous unilateral steps.”

“Those steps look increasingly as part of efforts to create a quasi-state on a large part of Syrian territory — from the eastern bank of the Euphrates River all the way to the border with Iraq,” he said.

The state Tass news agency on Tuesday cited Natalya Krylova, a municipal lawmaker in the town of Asbestos in the Urals, as saying local residents Igor Kosoturov and Stanislav Matveyev were killed in Syria Wednesday.

The Interfax news agency said that the Cossack group in the westernmost Kalningrad region said a member named Vladimir Loginov was killed in combat in the province of Deir el-Zour. It also quoted Alexander Averin, leader of extreme leftist group Another Russia, as saying that one of its activists, Kirill Ananyev, was killed by the U.S. strike near Khusham.

Russian media also cited unconfirmed claims that overall casualties could have been as high as 200 and Russians could have accounted for the bulk of them. Those claims couldn’t be verified.

Liberal politician Grigory Yavlinsky, who is running for president in Russia’s March 18 presidential election, urged Putin to come into the open regarding the deaths of Russians in Syria, saying that official silence on the subject was “unacceptable.”

“If many Russian citizens were killed, officials, including the commander in chief, must tell the nation about it and determine who is responsible for that,” he said.

Russia has waged a military campaign in Syria since September 2015, helping Syrian President Bashar Assad’s government seize large chunks of territory and turn the tide of war into its favor.

Russian media have reported that in addition to Russian servicemen deployed to Syria, thousands of Russians also fought there as private contractors, most serving with the so-called Wagner group. The private fighters allowed the Kremlin to keep the official death toll from its campaign in Syria low and avoid the negative public perception of its involvement in Syria as Putin is running for re-election in the March 18 vote.

The Kremlin and the Russian Defense Ministry refused to comment on media reports on Russian private contractors in Syria.

While the U.S. has used private contractors in Iraq and Afghanistan for years, Russian law forbids the hiring of mercenaries or working as one even though Russia has used proxies in eastern Ukraine, where it has backed pro-Moscow separatists since 2014. Some Russian lawmakers have called for legislation that would set rules for private military contractors.