Al Qaeda 'very active' in Afghanistan: U.S. Commander

Kabul, Afghanistan (CNN)Afghanistan's top defense official has warned that al Qaeda -- the reason the United States first invaded Afghanistan -- is "very active" and a "big threat" in the country.

A senior U.S. official said they were concerned about al Qaeda leaders in remote areas of the country and there may be many more core operatives in Afghanistan than previously thought.

The warnings of al Qaeda's resurgence come as Afghanistan faces perhaps the most significant summer fighting season in decades, with government security forces facing huge internal challenges, the Taliban both gaining ground and building links to al Qaeda, and ISIS increasing its footprint in the country.

Al Qaeda branch calls for new attacks against United States

Acting Defense Minister Masoom Stanikzai told CNN that al Qaeda were keeping a low profile but expanding.

"They are really very active. They are working in quiet and reorganizing themselves and preparing themselves for bigger attacks," he said.

"They are working behind other networks, giving them support and the experience they had in different places. And double their resources and recruitment and other things. That is how -- they are not talking too much. They are not making press statements. It is a big threat."

Major General Jeff Buchanan, Deputy Chief of Staff for the U.S. military presence in Afghanistan, said the recent discovery and destruction of an al Qaeda training camp in Kandahar province meant previous U.S. estimates of the group's strength were being revised.

"If you go back to last year, there were a lot of intel estimates that said within Afghanistan al Qaeda probably has 50 to 100 members, but in this one camp we found more than 150."

He admitted there was a gap in U.S. knowledge of the problem, and warned the group's core focus was still attacking the West.

"There's not thousands of them, but clearly in remote parts of Afghanistan there are al Qaeda leaders we're concerned about and what they're capable of doing."

U.S. officials said the number of core al Qaeda operatives in Afghanistan could be as high as 300, but that number does include other facilitators and sympathizers in their network.