BAGHDAD — After six weeks of American airstrikes, the Iraqi government’s forces have scarcely budged the Sunni extremists of the Islamic State from their hold on more than a quarter of the country, in part because many critical Sunni tribes remain on the sidelines.

Although the airstrikes appear to have stopped the extremists’ march toward Baghdad, the Islamic State is still dealing humiliating blows to the Iraqi Army. On Monday, the government acknowledged that it had lost control of the small town of Sichar and lost contact with several hundred of its soldiers who had been besieged for nearly a week at a camp north of the Islamic State stronghold of Falluja, in Anbar Province.

By midday, there were reports that hundreds of soldiers had been killed there in battle or mass executions. Ali Bedairi, a lawmaker from the governing alliance, said more than 300 soldiers had died after the loss of the base, Camp Saqlawiya. The prime minister ordered the arrest of the responsible officers, although a military spokesman put the death toll at just 40 and said 68 were missing.

“They did not have any food, and they were starving for four days,” a soldier who said he was one of 200 who managed to escape said in a videotaped statement that he circulated online. “We drank salty water; we could not even run.”