WASHINGTON -- It's a scenario many feared in the fog of Syria's multi-front war: a confrontation in which U.S. forces, responding to a provocation, kill Russian soldiers or mercenaries on a crowded battlefield. Russian news reports Tuesday described just such a scenario, with an unknown number of Russian military contractors killed in a ferocious U.S. counterattack last week.

But Defense Secretary Jim Mattis and other U.S. officials said they had no such information on casualties, and the Kremlin did not confirm any Russian deaths. U.S. officials also said the Russian government had lodged no complaint about its citizens being killed.

CBS News national security correspondent David Martin reported last week that Russian mercenaries were among the pro-Syrian regime forces hit by the strikes, and if any Russian nationals were among those killed, it would mark the first time a U.S. airstrike has killed Russians in Syria.

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What is not disputed is the fast-changing, often confusing nature of a battlefield in which forces of multiple countries are bumping up against one another, raising the prospect of violent collisions. Whether by accident or intention, such clashes risk plunging Washington and Moscow into a situation they studiously avoided even during some of the darkest hours of their relationship: their forces directly warring with each other.

Russian forces are supporting the Syrian government in its war with opposition groups, some of which are backed by the United States. Elements of both sides are fighting the last remnants of the Islamic State of Iraq and Syria (ISIS) group in Syria, and U.S. and Russian military officials maintain daily contact to avoid battlefield mishaps.

Beyond doubt is the ferocious scale of the U.S. attack on Feb. 7, in response to what the Pentagon called a barrage of artillery and tank fire from several hundred "pro-regime" fighters in Deir el-Zour province, an area in eastern Syria where the last ISIS fighters have converged among oil fields. Lt. Gen. Jeffrey Harrigian, commander of U.S. air forces in the Middle East, told reporters a broad range of U.S. air power was unleashed.

For more than three hours, American F-15E attack planes, B-52 strategic bombers, AC-130 gunships, Apache attack helicopters and Reaper drones fired on the attacking ground force, which Harrigian said was advancing under covering fire from artillery, mortars, rockets and tank rounds. The air power stopped the attackers' advance and destroyed an unspecified number of artillery guns and battle tanks, he said, but gave no estimate of casualties or full picture of the assailants.

"As the hostile forces turned west and retreated, we ceased fire," Harrigian, speaking from his headquarters in Qatar, said in a video teleconference with reporters at the Pentagon.

In a second episode, the U.S. struck a Russian-made T-72 battle tank on Saturday after it "took a shot at us" in the same general area of Deir el-Zour province, Harrigian said, adding that he did not know who was operating the tank.

Russian media said Russian private contractors were part of pro-Syrian government forces that advanced on oil fields in the Deir el-Zour province and were targeted by the United States. The reports cited activists who said that at least four Russian citizens were killed in Syria on Feb. 7.

The Russian Defense Ministry charged that the incident reflected a U.S. push to grab Syria's economic assets under the cover of fighting ISIS.

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

"Those steps look increasingly like 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 Russian state news agency Tass on Tuesday cited Natalya Krylova, a municipal lawmaker in the town of Asbestos in the Ural Mountains, saying local residents Igor Kosoturov and Stanislav Matveyev were killed in Syria on Feb. 7. The Interfax news agency reported that a Cossack group in the westernmost Kalningrad region said a member named Vladimir Loginov was killed in combat in the Deir el-Zour province. It also quoted Alexander Averin, the leader of extreme leftist group Another Russia, saying that one of its activists, Kirill Ananyev, was killed by the U.S. strike near Khusham, where the U.S. said its counterattack occurred.

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.

CIA Director Mike Pompeo listens to a question during and event on intelligence issues at the American Enterprise Institute in Washington, Tuesday, Jan. 23, 2018. AP

Asked at a Senate Intelligence Committee hearing about Russian deaths, CIA Director Mike Pompeo said, "From an intelligence perspective, we have seen in multiple instances foreign forces using mercenaries in battles that will begin to approach the United States." He deferred on the specifics on the incident to the Pentagon.

Mattis, speaking to reporters Tuesday while traveling in Europe, was adamant he knew of no Russian contractors killed in the fighting, which he attributed to a surprising assault in light of obvious U.S. advantages, including overwhelming air power.

"I don't have any reporting" about Russians being among the casualties, Mattis said. "I can't give you anything on that. We have not received that word" at key U.S. military headquarters, including the Pentagon.

A source from the Russian Foreign Ministry said Wednesday, "Reports on hundreds, dozens (of) fatalities in Syria disseminated by some Western media outlets are classic disinformation."

On Wednesday, Chair of the Russian State Duma Defense committee Vladimir Shamanov also told reporters that the reports were fake.