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.”

Here in Stuttgart, officials at the headquarters of United States Africa Command offered their own bleak assessment of a corruption-plagued, poorly equipped Nigerian military that is “in tatters” as it confronts an enemy that now controls about 20 percent of the country.

“Ounce for ounce, Boko Haram is equal to if not better than the Nigerian military,” said one American official here, who spoke on condition of anonymity to discuss operational reports.

The violence is in the meantime spilling into neighboring countries like Cameroon, which carried out its first airstrikes against Boko Haram this week, after militants overran a military base and attacked five villages there. Despite Boko Haram’s advances, United States Embassy officials in Abuja said Nigeria had canceled the last stage of American training of a newly created Nigerian Army battalion.

The United States’ original effort to help locate and rescue the girls produced scant results, American and Nigerian officials said, in part because of distrust. Although the United States reached an agreement with Nigeria last spring to share some intelligence, American officials did not include raw intelligence data because they believe that Boko Haram has infiltrated the Nigerian security services.

The United States has flown several hundred surveillance drone flights over the vast, densely forested regions in the northeast where the girls were seized, but officials in Stuttgart said that with few tips to guide the missions, the flights yielded little information, while diverting drones from other missions in war zones like Iraq and Syria.

When the Pentagon did come up with what it calls “actionable intelligence” from the drone flights — for example, information that might have indicated the location of some of the girls — and turned it over to the Nigerian commanders to pursue, they did nothing with the information, Africa Command officials said.

In addition, United States security assistance to Nigeria has been sharply limited by American legal prohibitions against close dealings with foreign militaries that have engaged in human rights abuses.

Last summer, the United States blocked the sale of American-made Cobra attack helicopters to Nigeria from Israel, amid concerns in Washington about Nigeria’s ability to use and maintain that type of helicopter in its effort against Boko Haram, and continuing worries about Nigeria’s protection of civilians when conducting military operations.

Those restrictions have drawn sharp criticism from Nigerian officials. In a speech at the Council on Foreign Relations in New York in November, Nigeria’s ambassador to the United States, Adebowale Ibidapo Adefuye, said his government was dissatisfied with the “scope, nature and content” of American support in the fight against Boko Haram. He also disputed allegations of human rights violations committed by Nigerian soldiers.

“We find it difficult to understand how and why in spite of the U.S. presence in Nigeria with their sophisticated military technology, Boko Haram should be expanding and becoming more deadly,” he said.

Mr. Adefuye accused Washington of failing to provide the lethal weapons needed to defeat Boko Haram. In June, the Pentagon gave Nigeria some Toyota trucks, communications equipment and body armor. “There is no use giving us the type of support that enables us to deliver light jabs to the terrorists when what we need to give them is the killer punch,” the ambassador said.

Mr. Adefuye’s speech prompted a strong response from the State Department the next day. “We continue to urge Nigeria to investigate allegations of abuses perpetrated by Nigerian security forces, as well as offer Nigeria assistance in developing the doctrine and training needed to improve the military’s effectiveness,” Jen Psaki, the State Department spokeswoman, told reporters in Washington. “We wouldn’t be raising that concern if we didn’t feel and others didn’t feel that they were warranted.”

Groups like Human Rights Watch say the Nigerian military has at times burned hundreds of homes and committed other abuses as it battled Boko Haram and its presumed supporters.

By this time, cooperation on the ground was also wearing thin. When Maj. Gen. James B. Linder, the head of American Special Operations forces in Africa, visited Nigeria in late October, he was barred from visiting the base where American trainers were instructing the new Nigerian Army battalion created to help fight Boko Haram. General Linder was left waiting at the gate in what some American officials viewed as another dig at the Pentagon. Africa Command officials insisted it was a “coordination issue that was remedied with a meeting later in the day.

“We continue to engage with Nigeria on a broad range of training, equipping, and information-sharing projects across all of the military services,” Benjamin Benson, an Africa Command spokesman, said in an email.

Secretary of State John Kerry called Nigeria’s president, Goodluck Jonathan, on Tuesday in part to discuss Boko Haram.

The strains between the two militaries are not new, and with Nigeria preparing for national elections in February, American officials fear that earlier assessments may overtake their cautious optimism from the spring.

Testifying before House and Senate hearings, administration officials in May offered an unusually candid criticism of the Nigerian military. “We’re now looking at a military force that’s, quite frankly, becoming afraid to even engage,” said Alice Friend, the Pentagon’s principal director for African affairs at the time.

Sarah Sewall, the undersecretary of state for civilian security, democracy and human rights, said at a separate hearing that despite Nigeria’s $5.8 billion security budget for 2014, “corruption prevents supplies as basic as bullets and transport vehicles from reaching the front lines of the struggle against Boko Haram.”

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