The Pentagon said the incident was in response to 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.
Lieutenant Harrigian was careful to avoid speculating who was inside the tank, though the T-72 is widely used by the Syrian army.
Russian Federation officially withdrew regular forces from Syria in December after two years of fighting in support of President Bashar al-Assad, but hundreds of private military contractors and mercenaries remained.
Vitaly Naumkin, a leading expert on the Middle East who has collaborated closely with Russia's Foreign Ministry on Syria, was somewhat more frank.
Harrigian said the US -backed coalition contacted the Russians through the "deconfliction" telephone line to make them aware of the unprovoked attack.
Briefing reporters by video at the Pentagon, Lt. Gen. Jeffrey Harrigian, who heads Air Forces Central Command, said that "deconfliction" conversations between the US and Russian militaries in Syria continued on a daily basis.
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Those released included several prominent opposition leaders and well-known journalist Eskinder Nega, a critic of the government. The board included it would discharge more points of interest on Saturday.
Confirming the fatalities on Thursday, Russian Foreign Ministry spokesperson Maria Zakharova said only that five people, "presumably Russian citizens", had been killed, "as a result of the armed conflict, the reasons for which are being clarified". And US and Russian military officials maintain daily contact to avoid battlefield mishaps.
On the eve the American television channel CNN published a video of the airstrikes of the global anti-terrorist coalition backed by the United States on the positions of Bashar al-Assad's forces in the Syrian region of Deir Ezzor.
Both sides are fighting against the last remnants of ISIS, which was mostly destroyed by the United States in the a year ago.
"The Russians professed that they were not aware when we called them about that force that had crossed [the Euphrates]".
The Russian workers had been at the site when "regime forces were trying to take the Coneco gas field" controlled by the Syrian Democratic Forces (SDF), a US-backed alliance of Arab and Kurdish fighters.
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The US reportedly accepted the claim.
The U.S. counterattack involved the use of F-15 fighter jets, B-52 bombers, AC-130 gunships and Apache combat helicopters as well as MQ-9 Reaper drones. "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. 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 US air forces in the Middle East.
The attack comes just days after USA forces for several hours used drones and B-52 bombers against several hundred "pro-regime" fighters equipped with artillery and a tank in Deir el-Zour province, an area in eastern Syria where the last IS fighters have converged among oil fields.
Dmitry Peskov, the spokesman for Russian president Vladimir Putin, refused to comment on reports of Russian casualties.
"Coalition officials were in regular communication with Russian counterparts before, during and after the thwarted, unprovoked attack", U.S. Colonel Thomas F. Veale, a military spokesman, said in a statement.
"No one wants to start a world war over a volunteer or a mercenary who wasn't sent by the state and was hit by Americans", Vitaly Naumkin, a senior adviser to Russia's government on Syria, said in an interview.
Yury Barmin, a Middle East analyst at the Russian International Affairs Council, a think tank set up by the Kremlin, said Russia supports Assad's efforts to reclaim the "crucial" eastern region of Deir Ezzor to help fund his national reconstruction and reconciliation plan, which the USA opposes.
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But he said ministers in his Nationals party, the junior partner in the coalition government, would abide by the new rule. Joyce warned that any pressure on the Nationals to dump him "locks people in as they lock-in behind the leader".
The reports follow one from Reuters on Monday, where former associates of two of the Russians killed identified them to the news wire. Those claims could not be verified. "It is notable that the posts about their deaths emerged rather quickly - relatives and colleagues often learn of Russian mercenaries' deaths weeks or even months after the fact", the group said in a Facebook post.