WASHINGTON — Islamic State fighters used a sandstorm to help seize a critical military advantage in the early hours of the terrorist group’s attack on the provincial Iraqi capital of Ramadi last week, helping to set in motion an assault that forced Iraqi security forces to flee, current and former American officials said Monday.

The sandstorm delayed American warplanes and kept them from launching airstrikes to help the Iraqi forces, as the Islamic State fighters evidently anticipated. The fighters used the time to carry out a series of car bombings followed by a wave of ground attacks in and around the city that eventually overwhelmed the American-backed Iraqi forces.

Once the storm subsided, Islamic State and Iraqi forces were intermingled in heavy combat in many areas, making it difficult for allied pilots to distinguish friend or foe, the officials said. By that point, the militants had gained an operational momentum that could not be reversed.

“The dust storm at the very least neutralized capabilities that could have been decisive,” said one former senior military official, who spoke on the condition of anonymity to discuss confidential battle assessments.