President George W Bush has been kept fully briefed about the situation in Pakistan and rise in violence across the border with India since the exit former Pakistan president Pervez Musharraf.

"Yes, the president is kept fully briefed on it," White House spokesperson Dana Perino told reporters Thursday when asked if Bush was watching or been briefed about the situation in Pakistan since the exit of it key ally in Islamabad.

"And I would refer you to Department of Defence, who would have more on their recent conversations with their military," she said.

Perino's comments came as the Pentagon confirmed that top US military officials had met the Pakistani Army's chief of staff, General Ashfaq Parvez Kayani, on August 26 on board the USS Abraham Lincoln in the Indian Ocean to continue "their ongoing dialogue about the war on terrorism."

Chairman of the Joint Chiefs of Staff, Admiral Mike Mullen, described his meeting with Kayani as "constructive and focused on the challenges posed by extremists in the federally administered tribal area and the North West Frontier in Pakistan."

Kayani understands the threat extremists pose to his country, Mullen told reporters at the Pentagon Thursday. The US and Pakistani top brass went over the specifics of the threat facing Pakistan and Afghanistan and what can be done about it.

"The Pakistani military faces a conventional military challenge from India and the extremist challenge," he said. "Kayani understands the situation and is moving toward combating the extremist problem on the border with Afghanistan."

"I'm pleased that he's moving in that direction and that he is, actually, operating," Mullen said. "And again, we're trying to figure out how that fits into bringing pressure onto that border to work to minimise the cross-border operations from Pakistan into Afghanistan on the case of the insurgents. It's just going to take some time."

"There is a growing complexity and coordination among extremist groups there - an almost syndicate-like behaviour - that has resulted in new and ever more sophisticated attacks on coalition forces," Mullen said referring to the tribal area that the Taliban and al-Qaeda are using to plan and train for attacks in Afghanistan.

Pointing to attacks against French forces near Kabul last week and against US forces in the Wanat Valley near Afghanistan's border with Pakistan last month, he said: "The safe havens in the border regions provide launching pads for these sorts of attacks, and they need to be shut down."

Accompanying Mullen at the conference with the Pakistanis was Army General David D McKiernan, commander of NATO forces in Afghanistan; Lt Gen Martin E Dempsey, acting US Central Command chief; General David H Petraeus, the soon-to-be commander of US Central Command who now commands Multinational Force in Iraq; and Navy Admiral Eric T Olson, commander of US Special Operations Command.

Mullen said he came away from the long-planned meeting "very encouraged that the focus is where it needs to be and that the military-to-military relationship we are building with Pakistan is getting stronger everyday."

"For me, more than anything, this was a chance to better understand a very complex challenge in a critical part of the world and to try to do that through the eyes of the leadership who live and work and fight there every single day," said Mullen who has met Kayani five times since February.

The meeting was important in "terms of learning as well as continuing to look at where we can support and how we can understand each other better, with a...very clear need from the United States' standpoint and from the Pakistani standpoint, that we have got to figure out a way to get at this problem," he said.

Different areas of Pakistan's border with Afghanistan pose different challenges, Mullen said, and long-term solutions must be in place to address the root causes of extremism.

"It continues to be an extraordinarily complex problem [in Pakistan]," he said. "We need to continue to press on it. There are areas that we can do better. There are areas that the Pakistan military can do better. We understand that. It's an area, I think, we can all improve on. But it is not going to be something that gets solved overnight."

The United States will continue to work with Kayani and will continue to reach out to improve the military-to-military relationship, he said.