WASHINGTON -- U.S. officials say there have been about 20 airstrikes against Islamic State of Iraq and Syria (ISIS) militants in eastern Afghanistan in the past three weeks, as the U.S. expands its fight against the insurgent group beyond Iraq and Syria.

The strikes come in the wake of a decision by the Obama administration to give the Pentagon the authority to conduct airstrikes against ISIS in Afghanistan. The group's numbers there continue to grow.

Army Brig. Gen. Wilson A. Shoffner told reporters that 1,000 to 3,000 ISIS fighters in eastern Afghanistan are trying to establish a base of operations in the rugged mountains of Nangahar Province.

Get Breaking News Delivered to Your Inbox

He declined to say how many airstrikes the U.S. has conducted in the east, but other U.S. officials said it was around 20. The officials weren't authorized to discuss the numbers publicly so spoke on condition of anonymity.

In a teleconference from Afghanistan, Shoffner said that ISIS militants have been launching attacks in the east but don't have the ability to conduct operations at more than one location at a time. He said that ISIS leaders in Iraq and Syria haven't yet been able to orchestrate any of the Afghanistan attacks.

Most of the ISIS fighters are from other groups that are simply rebranding themselves as ISIS, and Shoffner said many are former members of the Pakistani Taliban, known as Tehrik-e Taliban.

While ISIS has been a problem in eastern Afghanistan, U.S. and coalition efforts are more focused on the volatile south, where the Taliban have been launching persistent attacks since October in an effort to regain their heartland and protect smuggling routes for drugs and arms.

According to the U.S. officials, the number of American troops in Helmand Province is increasing from about 350 to 500. The officials said the U.S. is sending a few dozen more advisers to the region, but the bulk of the increase will be for forces to provide security for those trainers. The overall troop level for Afghanistan, which has been around 9,800, won't change.

Shoffner said the additional advisers will help rebuild the Afghan's 215th Corps and special operations forces, which launched a counteroffensive against the Taliban in November.

He said the Afghans have increased the number of special operations forces in Helmand in an effort to take some of the pressure off the 215th Corps, which needs time to reset and retrain.

The U.S. has outlined four areas that the Afghans need to work on in the coming months, as the military continues to review Obama administration plans that would reduce the American presence in the country to about 5,500 by the end of the year.

Shoffer said the Afghans need to increase recruiting to fill a shortfall of about 25,000 troops in the Army. The Army is training up to 4,000 recruits a month, far short of its 6,000 goal.

Afghan forces also need time to retrain and rest between fighting deployments, and should replace weak leaders, he said. The fourth key change, he said, would be to reduce the number of checkpoints that Afghan security forces are staffing. Too many of the checkpoints can be overtaken by the Taliban, so the Afghans should readjust by focusing on fewer checkpoints that they can effectively control, Shoffer said.

Asked when the U.S. would begin withdrawing troops to meet the 5,500 goal by year's end, Shoffer said discussions are ongoing, but the reduction is small enough that it will be "much less of a challenge" than previous drawdowns.

At the war's peak, the U.S. had about 100,000 forces in Afghanistan.