"Afghanistan is very vulnerable in terms of Taliban and extremists taking over again, and I don't think that threat is going to go away. They still plot against us," Admiral Mike Mullen, Chairman, Joint Chief of Staff told the NBC news channel in an interview.
"(They) still very capable, very focused on it, the leadership is. They also are able to both train and support and finance, and so that capability is still significant and one which we are very focused on making sure that (9/11) doesn't happen again," he said.
Appearing on Meet the Press programme, Mullen said, they are linked, they have a safe haven in Pakistan.
"Across that border in Pakistan, they provide the safe haven for Al Qaeda. They also feed fires into Afghanistan. Al Qaeda would very much like to see Kabul become the capital that it was before essentially run by extremists".
In a separate interview to the CNN, Mullen said the situation in Afghanistan "is serious and it is deteriorating, and I have said that over the last couple of years, the Taliban insurgency has gotten better, more sophisticated."
"Their tactics in my recent visits out there and talking with our troops certainly indicate that," he said.
The top US military officer said the US military is "very focused on defeating Al Qaeda and the Taliban network in Afghanistan and Pakistan and making sure that it doesn't happen again," referring to the 9/11 attacks.
He said, "They see us as something they want to kill in terms of as many American lives as possible in that regard. We are very focused on executing that mission."
"I am very mindful and concerned about the threat that's there; the strategy really focuses on defeating Al Qaeda and their extremist allies. That's where the original 9/11 attacks came from that region. They have now moved to Pakistan," the Admiral said.
Noting that the mission the President gave to the US army is to defeat and disrupt Al Qaeda and its extremist allies, he said, "That's very specific, and that includes the Taliban, which has grown to be much more sophisticated in the last two to three years and is a much tougher enemy in that regard."
"So, in that regard, it's very much linked and, again, it's the mission that the military has right now to focus -- and General McChrystal is doing this -- focus on the security for the people, focus on the Afghan people," he said.
Responding to a question, Mullen said a fresh assessment of current situation in Afghanistan is going on and no decision has been made by the North Atlantic Treaty Organisation / US commander there on the increase in number of troops.
"We are not at a point yet where he's (NATO/US commander) made any decisions about asking for additional troops," Mullen said.
He said "From military perspective, I believe we have got to start to turn this thing around from a security standpoint in the next 12 to 18 months."
"I think after that we'd have a better view of how long it's going to take and what we need to do. We are just getting the pieces in place from the President's new strategy in March on the ground now both on the military side we have put forces there, and we will add more this year -- and on the civilian side. So it is going to take us a while to understand that," he pointed out.
Appearing on the CNN talk show, the US Ambassador to Afghanistan, Lt Gen (rtd) Karl Eikenberry, said, "We have a very difficult situation in parts of Afghanistan today."
However, he added, "What we do have for the first time, I believe, since 2002, we have a very clear strategy, and matched against that we have sufficient -- we have resources that are being mobilised."