WASHINGTON — The United States has significantly intensified its bombing campaign in Afghanistan in the past two months as part of President Obama’s widening war against the Islamic State militants who have seized territory outside of Iraq and Syria, according to senior military commanders.

American drones and fighter jets dropped 251 bombs and missiles in January and February in Afghanistan, more than three times the strikes in the same period last year, according to data compiled by the Air Force.

The strikes came in response to a decision by Mr. Obama around the beginning of the year that gave the military more leeway to launch attacks on Islamic State militants who had gained control over territory in several provinces, including areas in the Tora Bora region, where Osama bin Laden once took refuge.

Afghan and American commanders said that while the strikes have dealt a blow to the Islamic State, they have broader concerns about the security situation in Afghanistan because the Taliban appear stronger than at any point since 2001, and its 20,000 to 40,000 fighters are estimated to be at least 20 times the number of militants aligned with the Islamic State.