At the same time, media reports emerged the day before, alleging that a large number of Russian mercenaries had been killed in airstrikes.
Syrian government forces in Deir Ezzor.
Initially, the Syrian opposition refused to participate in the Sochi talks opting instead to further the United Nations -led Geneva talks. Russia, Iran, Israel and Turkey have all had aircraft shot down in or near Syria this month.
On Jan. 30 participants at the Congress agreed to set up a committee to rewrite the Syrian constitution in Geneva and to hold democratic elections in Syria. In December previous year, President Vladimir Putin ordered the partial retreat of Russian troops from Syria.
The mercenaries are said to have been hired by private military firms backing Syrian pro-government troops.
Dmitry Peskov, Russian President Vladimir Putin's spokesman, did not comment on the reports of Russian deaths, saying the Kremlin only tracks data on Russia's armed forces.
For reasons that remain unclear, Syrian government troops and some Russian nationals appear to have attacked a coalition position, near Al Tabiyeh, Syria.
Mevlut Cavusoglu also told reporters in Istanbul that ties between Turkey and the US are at a "very critical stage", the Associated Press said.
What happened in the United States strikes?
On Tuesday, US Central Command said US forces came under fire from a tank, and that they responded with strikes from drones and B-52 bombers against the attackers. The Guardian reported air force Lt Gen Jeffrey Harrigan as saying that US advisers had been in an operating base when they came under tank fire.
And responding to a further BBC question, the spokesman said: " And, of course, you have to realise that many countries of the world have quite large numbers of our compatriots - of Russians - and, of course, it's hard to have detailed data.
But it comes after news hundreds of Russian mercenaries were reportedly killed by US-backed forces last week.
USA military officials also said that, during the air assault, they had been in contact with the Russian military, via a special hotline set up by the two sides to avoid hostile incidents between the two.
"I don't have any reporting - that some Russians - non-Russian federation soldiers, but Russian contractors were among the casualties".
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.
Russian forces are supporting the Syrian government in its war with opposition groups while staying in daily contact with the U.S.to avoid deadly battlefield collisions.
Vladimir Frolov, a security expert and former Kremlin adviser, told The Independent Russia nearly certainly knew about Wagner's participation in the Syrian-led attack.
Presidents Putin and Trump and Putin spoke by phone in an unscheduled call on Monday, and it is likely that the issue was discussed. It's also the most visible manifestation of American-on-Russian violence, even if the erstwhile Cold War adversaries were not technically engaged against each other.