Islamic State fighters trapped in Syria are putting up fierce resistance, using civilians as human shields, setting deadly booby traps, and hiding in underground tunnels, all the while frustrating efforts by U.S.-backed Syrian fighters to deal a final blow to the terrorist caliphate.

It’s been six months since the start of “Operation Roundup,” an offensive designed to liberate the last swath of territory held by ISIS in the Middle Euphrates River Valley, or MERV, near Syria’s eastern border with Iraq.

“ISIS, however, remains a deadly adversary. The remaining fighters in the MERV are hardened combatants and have shown every indication of being willing to fight until the end,” Army Col. Sean Ryan told Pentagon reporters in a briefing from Baghdad.

The ruthless tactics of the remaining ISIS fighters have slowed the advance of the Syrian Democratic Forces, which has to be careful to protect the civilian populations who are often held hostage in their homes and prevented by ISIS from leaving the battle zone.

“We're methodically going through that area, making sure they capture or kill all the ISIS fighters,” said Ryan, adding “our job there is to also help protect its citizens in that area, and I think the SDF's doing a pretty good job.”

For months, the U.S. has said ISIS fighters are essentially trapped and taking heavy losses.

An estimated 700 fighters and their families have surrendered and are in the custody of the SDF, but the remaining fighters show no sign of giving up.

“They're a resilient enemy. There's no doubt about that,” said Ryan, who says the last remnants of ISIS knew this is where they would make the last stand and prepared to fight to the end.

“With the tunnels they have underneath and the tunnels that the oil companies left with food and supplies, they're able to sustain,” he said.

When the offensive began in May, the U.S. estimated ISIS held only 2 percent of the territory it once controlled.

In the past six months, that estimate has not been changed, although the SDF has cleared a few towns.

“It's not about the land mass; it's about taking away ISIS capabilities,” Ryan said. “We're degrading them every day. It's not just killing ISIS fighters; it's taking away their weapons systems, taking away their logistical support and things of that nature. So, that's happening every day.”

Asked by a reporter if it looked as though the offensive, which was expected to last a few months, would extend into next year, Ryan said, “We've still got about three months to go, and a lot can happen in 2018.”