WASHINGTON (Reuters) - The United States has “significantly” increased its air strikes against Islamic State in Afghanistan since President Barack Obama granted commanders broader authority last month to target the group there, a U.S. military spokesman said on Thursday.

Islamic State is a relatively new force in Afghanistan and the militant group has violently challenged the much larger Afghan Taliban movement in pockets of the country.

Obama granted the U.S. military the authority to strike Islamic State, also known as ISIS, ISIL, or Daesh, in January, with concerns mounting about the group’s increasing global reach. U.S. forces could previously strike Islamic State in Afghanistan but it was under more narrow circumstances, such as for protection of troops.

“We have significantly increased our pressure and the number of strikes we’ve conducted against Daesh in Nangarhar province over the past three weeks,” Brigadier General Wilson Shoffner, a U.S. military spokesman, said in a briefing to reporters. “The change in authorities has given us additional flexibility.”

Shoffner declined to give specifics on the number of strikes but said combined with pressure from Afghan security forces, they had succeeded in containing Islamic State in the southern part of Nangarhar province, in eastern Afghanistan. According to the Afghan interior ministry, Afghan and international forces conducted nearly 20 joint operations against Islamic State in Nangarhar in January.

Islamic State is not yet able to conduct operations in more than one place at a time in Afghanistan, Shoffner said, but was attempting to establish a base of operations in Nangarhar and carry out low-level recruiting in various parts of the country. In eastern Afghanistan, Islamic State numbers some 1,000 to 3,000 members, he said.

Those joining Islamic State are typically former members of the Afghan Taliban or the separate Pakistani Taliban, known as TTP, Shoffner said. Nangarhar borders the Federally Administered Tribal Areas of Pakistan.

“In terms of motivation what we see are generally former TTP who believe that associating with Daesh or pledging to Daesh will further their interests in some way,” he said.

NATO aircraft struck an Islamic State radio station in Nangarhar this month. The station had been broadcasting up to 90 minutes a day in the Dari and Pashto languages.