Americans are fighting near Russians in parts of Syria while working together with the Kurdish YPG fighters against the Islamic State.
While commanders for both the United States and Russia are in contact with each other, the Americans ended military-to-military cooperation when Russia annexed Crimea from Ukraine in 2014, the Daily Mail reported.
Army Lt. Gen. Steven Townsend, the commander of Combined Joint Task Force Operation Inherent Resolve, told reporters that soldiers from the two countries were within "hand-grenade range of one another."
Andreas Krieg, a professor at the Defense Studies Department at King's College London, told NBC News that tentative cooperation between the Americans and Russians with the Kurds serves "short and midterm interests" which are "are overlapping to a huge extent."
"Fighting ISIS and fighting the jihadis is absolutely the first priority of the (Donald) Trump administration," Krieg told NBC News. "This is why (Defense Secretary James) Mattis is going so hardcore after ISIS. And almost everything goes as long as they are fighting jihadis at the same time."
Krieg warned, though, that the close proximity fighting could lead to the Americans or Russian troops inadvertently hitting one another despite fighting on the same side, NBC News noted.
"Escalation is bound to happen," Krieg told NBC News.
While both are fighting ISIS, the United States and Russia are backing different sides in the Syrian civil war. The Russians are supporting the ruling Syrian government while Americans are supporting rebel fighters who are opposed to the government.
On March 1, Russian aircraft mistakenly bombed Syrian fighters trained by the United States, with American advisers just 3 miles away from the Russian strike, according to The New York Times.
"We had some Russian aircraft and regime aircraft bomb some villages that I believe they thought were held by ISIS," Townsend said in a video news conference at the Pentagon, according to the Times then. "Actually on the ground were some of our Syrian Arab coalition forces."
The incident illustrates the complicated, crowded battlefield in northern Syria, occupied by Turkish-backed Syrian militias, Syrian government forces, Kurdish and Arab fighters backed by the United States, ISIS militants, along with U.S. and Russian forces, the Times noted.