The U.S. military thanked Russia on Friday for intervening with the Syrian regime to ease tensions with U.S.-backed rebel forces on the Jordanian border.

"We asked the Russians to help," and the Russians have been "very helpful" in limiting clashes between the partnered force at the border town of Al Tanf and forces of the regime of Syrian President Bashar al-Assad backed by Hezbollah fighters, said Navy Capt. Jeff Davis, a Pentagon spokesman.

At a Pentagon briefing, Davis stressed that the U.S. had no direct contacts with the Syrian regime but used the so-called "deconfliction channels" with Russia to convey concerns over the situation on the border and warn that the U.S. retains the right to self-defense.

Davis said the border area was fairly calm Friday as opposed to Thursday, when a U.S. F-15 shot down a large unmanned aerial vehicle, or drone, that had dropped a munition on an area controlled by the U.S.-backed force. The munition failed to detonate.

U.S. aircraft also attacked and destroyed two vehicles that were headed to a camp set up by pro-regime forces, Davis said.

Army Col. Ryan Dillon, a U.S. military spokesman in Baghdad, said Thursday that the drone was "about the same size as our MQ-1 Predator. It was kind of like that." The suspicion is that the drone was supplied by Iran, which has supported the Assad regime with troops and supplies.

Davis would not confirm that the drone came from Iran but said, "We do know that Iran, obviously in its support for the regime, provides them with equipment, provides them with assistance and support with funding."

Davis said the U.S. acted in self-defense out of concern for U.S. troops in the area. "We're certainly not there to occupy and control" territory, he said. "We're there to defeat ISIS -- full stop."

On other issues, Davis said that the battle for Mosul, which entered its 110th day Friday, has turned into a bitter slog in the tight streets and alleyways of the old city on the western side of the city, which is split by the Tigris River.

Iraqi Security Forces are slowly advancing against the remnants of ISIS fighters at a rate of about 100 meters per day, Davis said. He did not have a timeline on when the northeastern Iraqi city would be liberated from more than two years under the control of the Islamic State of Iraq and Syria.

In Syria, the U.S.-backed Syrian Democratic Forces expanded a foothold gained earlier this week in Raqqa, the so-called ISIS capital in northeastern Syria. Davis said the SDF now control about one square kilometer in the eastern side of the city. He estimated that about 2,500 ISIS fighters are holding the city.

-- Richard Sisk can be reached at Richard.Sisk@Military.com.