U.S. President George Bush says his country can help "calm down" the strained relations between Afghanistan and Pakistan over insurgent attacks.

At a news conference Monday in London, Bush did not endorse the threat issued Sunday by Afghanistan's President Hamid Karzai to send troops into Pakistan to battle the Taliban.

Bush said the U.S.'s goal is to deny a safe haven to violent extremists.

"That's the strategy of Afghanistan. It needs to be the strategy of Pakistan," he said.

Relations between Pakistan and the U.S. were strained last week after U.S. air strikes left 11 Pakistan paramilitary troops dead during a border-area clash with insurgents.

In eastern Afghanistan on Monday, hundreds of Afghans rallied to support Karzai's threat.

"We are ready to sacrifice, like before, for the protection of our homeland borders," said Ghami Mohammad Yar. He is a spokesperson for the governor of Paktika province, which borders Pakistan.

Mohammad Akram Akhpelwak, Paktika's governor, said gatherings of support were being held in four areas of Paktika.

In Pakistan, Foreign Ministry spokesperson Mohammed Sadiq said Monday that the government would issue a formal response to Karzai.

He also added, "Naturally we think that he did not use his best judgment by making this statement."

A spokesperson for the second-largest party in Pakistan's coalition government condemned Karzai's position.

"Pakistan as a sovereign state will not permit any Karzai to violate the international border," said Sadiqul Farooq of the Pakistan Muslim League-N party of former prime minister Nawaz Sharif.

A spokesperson for Tehrik-e-Taliban Pakistan, Pakistan's Taliban movement, warned that Taliban attacks against NATO and Afghan forces would escalate if Karzai sent forces into Pakistan.

A spokesperson for the NATO-led International Security Assistance Force said he wouldn't comment.

One ISAF official, who asked not to be identified, told The Associated Press that Karzai's words shouldn't be interpreted to mean that an attack is imminent.

Instead, they should be seen as indicative of Karzai's ongoing frustration with the fact that the tribal regions of Pakistan continues to be a safe haven for the Taliban.

With files from The Associated Press