The assault was launched immediately after a base housing U.S.-backed fighters from the Syrian Democratic Forces and U.S. military advisers was attacked.

CIA Director Mike Pompeo has said that a "couple hundred Russians" were killed in eastern Syria during U.S. artillery and air strikes in early February, an attack that remains shrouded in mystery.

The comments by Pompeo, which came during his testimony before a Senate committee on April 12, was a rare open acknowledgment by a top U.S. official about the incident, RFE/RL reports.

"In Syria, now, a handful of weeks ago the Russians met their match. A couple hundred Russians were killed. The list of actions that this administration has taken... I'm happy to walk through each of them but I don't want to take up more time," he said.

Read alsoVideo released of U.S. massive airstrike on Russian mercenariesU.S. military commanders had previously said that a massive ground and air assault was launched on February 7 against a column of fighters and weaponry in the eastern Syrian province of Deir al-Zor.

The assault was launched immediately after a base housing U.S.-backed fighters from the Syrian Democratic Forces and U.S. military advisers was attacked.

U.S. officials earlier said more than 100 pro-government Syrian fighters had been killed, and that AC-130 gunships, F-15 and F-22 fighter jets, and Apache helicopter gunships had been used in the U.S. assault, along with Marine Corps artillery units.

No U.S. personnel were injured in the attack.

A few days after the attack, U.S. officials revealed that U.S. commanders had utilized a special "hot line" to communicate directly with their Russian counterparts, before, during, and after the attack.

Read alsoRussian military "advised" to quit social networksU.S. Defense Secretary Jim Mattis later called the attack "perplexing."

In Moscow, Russian officials said no uniformed Russian troops were involved in the attack.

Russian and Western news media, including RFE/RL, have sought out relatives of those killed, trying to tally the casualty figures.