WASHINGTON — The deteriorating situation in Iraq is giving Congress pause about President Barack Obama’s plan to withdraw U.S. forces from Afghanistan by the end of 2016, with fears that hard-fought gains could be wiped out by a resurgent Taliban.

Senior Obama administration officials insist Afghanistan is not Iraq, with a population far more receptive to a continued U.S. presence and the promise of a new unity government. But the officials could offer no assurances that Afghanistan won’t devolve into chaos after Americans leave, as Iraq has.

“There’s no guarantee,” Defense Secretary Chuck Hagel told a Senate panel Wednesday. “It is up to the people of Afghanistan to make these decisions, their military, their new leadership that will be coming in as a result of their new government.”

The U.S. military mission in Iraq ended in December 2011 after eight years of war that cost hundreds of billions of dollars and more than 4,400 U.S. lives, a conclusion welcomed by a war-weary nation. The Obama administration had proposed keeping a residual U.S. force in Iraq to continue training Iraqis, but Baghdad rejected Washington’s demand that its troops be granted immunity for prosecution while in the country.

In the absence of the Americans, the fast-moving Sunni insurgency of the Islamic State of Iraq and the Levant has prevailed over Iraqi security forces, conquering several cities, and is threatening the government of Prime Minister Nouri al-Maliki. Gen. Martin Dempsey, chairman of the Joint Chiefs of Staff, described for Congress on Wednesday how some Iraqi security forces abandoned the fight against the ISIL.