The discovery of the large camp attacked in October raised questions about the American military’s ability to detect and destroy a major Qaeda stronghold in the country, more than 14 years after the American-led invasion of Afghanistan drove out Al Qaeda and toppled the Taliban government that supported them.

General Campbell said at the time of the October raid that the camp was used by a new Qaeda offshoot called Al Qaeda in the Indian Subcontinent, or AQIS. Ayman al-Zawahri, Al Qaeda’s overall leader, announced the creation of the affiliate in September 2014 largely in response to the rise of its rival, the Islamic State, which is also known as ISIS or ISIL. The wing, which American analysts say has several hundred fighters, is believed to be based in Pakistan and focused on India, Pakistan and other nations in southern Asia.

AQIS fighters began migrating from sanctuaries in North Waziristan and eastern Afghanistan to the country’s southern provinces of Helmand and Kandahar last year, after Pakistan launched a military offensive in the region, said Seth Jones, an Afghanistan specialist at the RAND Corporation. Kandahar and Helmand have not typically been havens for Al Qaeda.

“It’s been a relatively recent expansion to the south,” Mr. Jones said.

Afghan security officials say many of these foreign fighters filtering in are from Central Asia, and in many cases their affiliations are unknown. In the past, some of the groups have been affiliated with Al Qaeda, but there have also been reports of some of these fighters pledging allegiance to the Islamic State.

One American intelligence official sought to play down the menace from the new Qaeda offshoot, calling it “a regional threat that is currently focusing on plotting attacks in Pakistan and establishing a presence in South Asia. Despite its safe haven, the group has not been seen conducting attacks against Afghan or Western targets in Afghanistan.”

The emergence of new Qaeda training camps comes amid a widespread erosion in security in much of the country. “In the second half of 2015, the overall security situation in Afghanistan deteriorated, with an increase in effective insurgent attacks and higher A.N.D.S.F. and Taliban casualties,” the Pentagon said in a report issued two weeks ago, using the initials for the Afghan National Defense and Security Forces.

General Campbell told lawmakers that the Pakistani-based militancy, the Haqqani network, remains an important “facilitator” for Al Qaeda in Afghanistan. The two groups, he said, share a goal of “expelling coalition forces, overthrowing the Afghan government, and re-establishing an extremist state.”