The additional troops will bring the official number of American service members deployed in Iraq to 4,647. Many will be based at an airfield about 40 miles south of Mosul that was seized by the Iraqis on Saturday.

“This contingent will help the Iraqis with a logistical springboard for their offensive in Mosul,” Mr. Carter said.

American commanders plan to use the base, Qayyarah West Airfield, as a staging area to provide logistical support to Iraqi forces as they move toward Mosul. Some of the troops specialize in infrastructure support like building bridges, which the Iraqis will need for the assault on Mosul because the Islamic State has destroyed many around the city. The Islamic State seized Mosul in June 2014.

Mr. Carter made the announcement while addressing troops at the international airport in Baghdad. Earlier in the day, Mr. Carter met with Iraq’s prime minister, Haider al-Abadi.