Secretary of Defense James Mattis explained Thursday why he directed a strike that reportedly killed hundreds of Russian mercenaries in Syria back in February.

Mattis told the Senate Armed Services Committee that the U.S. has a deconfliction line with Russia to ensure that the two countries can communicate in order to avoid direct conflict with one another in Syria. He said that a group of "irregular forces" were in conflict with U.S. forces, and once it was ascertained that those forces were not Russian regulars, Mattis directed a counterattack.

"The Russian high command in Syria assured us it was not their people, and my direction to the chairman was for the force, then, to be annihilated," Mattis said. "And it was."

The force comprised hundreds of Russian mercenaries, which then-CIA Director Mike Pompeo attested to when he said the U.S. killed "a couple hundred Russians." On Thursday, the Senate confirmed Pompeo to be President Donald Trump’s secretary of state.

Asked about whether the Russian Federation was harassing U.S. forces in Syria, Mattis stopped short of blaming Russia for particular battlefield actions.

"I cannot target the responsibility to the Russians right now," he said. "It is a crowded battlefield; it’s also got Iranians there and, of course, the regime forces as well."

He touted the sanctions the Trump administration has imposed on specific Russians.

"You notice as we go forward, we’ve so far sanctioned 189 individuals in Russia," he said.

"Economic sanctions are going to be obviously looked at for future violations as well," Mattis added. "So we have an asymmetric way, an indirect way, of going after them and making them pay."