Washington, Dec 16 : US President George W Bush has vouched for Pakistan President Asif Ali Zardari's anti-terror credentials, after the latter looked him straight in the eye and told him he did not need to be told about the horrors of terrorism.

"President (Asif Ali) Zardari is determined ... he's said so publicly and he's said so to me privately, Bush told reporters traveling with him on Monday.

"He looked me in the eye and said `you don't need to talk to me about extremist violence, after all, my wife got killed by extremists'," Bush added.

He acknowledged the significance of a regional co-operative approach to deal with terrorism when asked about plans to look at the problem in the regional context, the Daily Times reported.

He said the US was working with Pakistan to build pressure on the Taliban along the Pakistan-Afghanistan border.

"There needs to be a comprehensive strategy in helping the Pakistan government deal with those who bring great harm on their citizens," he said.

"The more that we can get Pakistan and Afghanistan to co-operate, the easier it will be to enforce that part of the border region," he said, according to a transcript released by the White House.

Bush was travelling to Afghanistan from Iraq on a surprise last visit to the insurgency-hit country.

In Afghanistan, Bush told Afghan President Hamid Karzai that the US would stand by the war-torn country despite a transition of power at the White House.

Bush appeared to lend tacit support to president-elect Barack Obama's pledge to increase troop levels in Afghanistan. (ANI)