If confirmed, the series of airstrikes would rank among the highest civilian death tolls in an American air mission since the United States went to war in Iraq in 2003. And the reports of civilian deaths in Mosul came immediately after two recent incidents in Syria, where the coalition is also battling the Islamic State from the air, in which activists and local residents said dozens of civilians had been killed.

Taken together, the surge of reported civilian deaths raised questions about whether once-strict rules of engagement meant to minimize civilian casualties were being relaxed under the Trump administration, which has vowed to fight the Islamic State more aggressively.

American military officials insisted on Friday that the rules of engagement had not changed. They acknowledged, however, that American airstrikes in Syria and Iraq had been heavier in an effort to press the Islamic State on multiple fronts.

Col. John J. Thomas, a spokesman for the United States Central Command, said that the military was seeking to determine whether the explosion in Mosul might have been prompted by an American or coalition airstrike, or was a bomb or booby trap placed by the Islamic State.