STUTTGART, Germany — Soon after the Islamist group Boko Haram kidnapped nearly 300 teenage girls in Nigeria in April, the United States sent surveillance drones and about 30 intelligence and security experts to help the Nigerian military try to rescue them. Gen. David M. Rodriguez, the top general for American missions in Africa, rushed from his headquarters here to help the commanders in the crisis.

Seven months later, the drone flights have dwindled, many of the advisers have gone home and not one of the kidnapped girls has been found. Many are believed to have been married off to Boko Haram fighters, who in the past six months have seized hundreds more civilians, including children, planted bombs in Nigerian cities and captured entire towns.

In Washington, that fleeting moment of cooperation between Nigeria and the United States in May has now devolved into finger-pointing and stoked the distrust between the two countries’ militaries. Nigeria’s ambassador to the United States has accused the Obama administration of failing to support the fight against Boko Haram, prompting the State Department to fire back with condemnations of the Nigerian military’s dismal human rights record.

“Tensions in the U.S.-Nigeria relationship are probably at their highest level in the past decade,” Johnnie Carson, the State Department’s former top diplomat for Africa, said in an interview. “There is a high degree of frustration on both sides. But this frustration should not be allowed to spin out of control.”