– The U.S.-led air campaign to hunt down the last pockets of ISIS militants in eastern Syria has effectively ground to a halt in the past two months after the allies lost their most effective battleground partner, stalling a critical phase of the offensive.

With ISIS fighters starting to claw back some of their lost territory in Syria, and with President Donald Trump previously threatening to withdraw U.S. troops there before finishing off the last militants holding ground, commanders have rushed to adopt new tactics to regain some momentum. (The president subsequently dropped his demand for an immediate withdrawal when commanders told him they needed time to successfully finish the mission.)

The new approach includes stacking several surveillance planes over two big remaining pockets of fighters, patiently watching the suspected enemy’s every move for days — and then striking only when sure the foes are really foes, and the risk to civilians is low. The new tactics have helped increase strikes in eastern Syria to 23 last week compared with only three in the week ending April 5, military officials said.

Of course, those figures pale in comparison to the nearly 400 strikes a week during the height of the air war last fall to seize Raqqa, the self-proclaimed ISIS capital. But U.S. officials say the latest attacks against bunkers, bomb factories and headquarters show the air campaign’s ability to adjust to an unforeseen setback that threatens to hand ISIS a lifeline just as the allies are on the verge of wiping out the last insurgent safe havens.

“The remaining numbers of ISIS fighters is less of a concern for us than it is the ability for them to stand up and work as networks and work as an organization,” Col. Ryan Dillon, spokesman for the U.S.-led coalition, said last week.

Throughout the monthslong air campaign, allied warplanes have relied mainly on Syrian Kurdish militia to flush out insurgents from their hideouts or fortified positions, or help pinpoint their locations. That served up targets for allied fighter-bombers. But those militia fighters started leaving eastern Syria in late January to defend other Kurds against Turkish attacks.

The Kurdish-led Syrian Democratic Forces were the mainstay in routing ISIS from Raqqa and chasing insurgents fleeing south along the Euphrates River Valley to the Iraqi border. Without them, the remaining, less capable Syrian Arab militiamen have struggled to contain the few hundred fighters left in two main areas: Hajin, along the Euphrates River north of Abu Kamal; and Dashisha, east of Deir el-Zour, along the Syria-Iraq border.

“That is where ISIS in eastern Syria is concentrated and that’s where we are going after them,” Dillon said.

And it’s not just an issue for the United States and other Western air forces. Using intelligence provided by the coalition, Iraqi fighter jets on April 19 attacked ISIS targets near Hajin that Iraqi and U.S. officials said threatened Iraqi security just across the border.

“Iraqi air forces strike #ISIS terrorists in eastern #Syria organizing to threaten western #Iraq,” Brett H. McGurk, the American presidential special envoy to the anti-ISIS coalition, said in a tweet. “More surprises in store in these few remaining ISIS havens.”