The U.S. military says that while the fight against the Islamic State continues in Syria, Russian-backed Syrian government forces have no effective defenses to prevent remnants of the terror group from escaping to havens in regime-held areas in the west of the country.

While Russian President Vladimir Putin declared final victory over ISIS during a visit to Russian troops in Syria last week, the U.S. says the Syrian Democratic Forces are still in pitched battles with remnants of ISIS in the Middle Euphrates River Valley, or MERV, where as many as 2,000 ISIS fighters are staging their last stand.

“Despite recent claims from Russia that Syria is free from ISIS terror, the SDF are still meeting resistance from ISIS fighters in the region,” said Col. Ryan Dillon, a U.S. military spokesman who briefed Pentagon reporters via phone from Kuwait.

The U.S. describes the ISIS effort as a stalling tactic to allow hundreds of the last remaining fighters to escape to areas near the Syrian capital of Damascus, where they are not being pursued by forces loyal to Bashar Assad.

The remaining ISIS fighters, described by Dillon as “a desperate and dwindling ISIS force,” continue “to fight vigorously” to provide cover for other ISIS elements who are moving to southwestern and northwestern Syria to seek sanctuary or continue to fight.

“Syrian regime commanders in eastern Syria suggest that ISIS fighters from the MERV may have slipped through porous Syrian and Russian defenses to arrive in areas near Damascus,” Dillon said.

“We have also seen a recent ISIS attempt to transit westward through the An-Tanf area, albeit unsuccessfully because of our partners' actions to interdict.”

U.S. Central Command also announced that three senior ISIS leaders were killed in two separate airstrikes in the past three weeks.

On Dec. 1, Abu Faysal, described as a senior ISIS leader, and his deputy Abu Qudamah al-Iraqi, were killed in the Middle Euphrates River Valley region of Syria.

And on Nov. 28, a man described as a senior ISIS courier, Mustafa Kamal Jasim Muhammad al-Zawi, was killed near al-Sharqat, Iraq.

While the battle against ISIS continues in Syria, the U.S.-led counter-ISIS coalition has congratulated Iraq for what it called the “the full liberation from ISIS,” even as operations continue to mop up ISIS remnants who have gone underground.