WASHINGTON — The United States-led air campaign to hunt down the last pockets of Islamic State militants in eastern Syria 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 Islamic State fighters now starting to claw back some of their lost territory in Syria, and with President Trump previously threatening to withdraw American 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.