Tension is growing between the United States and its ally Pakistan over how to tackle al-Qaeda and Taliban militants as US forces in Afghanistan step up cross-border raids on militants in Pakistan.
US forces have launched about a dozen missile strikes from pilotless drones at militant targets in northwest Pakistan this year killing scores of suspected militants and some civilians.
But in a major intensification of US action in Pakistan, helicopter-borne US commandos carried out a ground assault in a Pakistani tribal region last week killing 20 people, including women and children, Pakistani officials said.
US Chairman of the Joint Chiefs of Staff Admiral Mike Mullen later said a "more comprehensive strategy" was being formed to combat the militant threat and the United States would revise its strategy to combat militant havens in Pakistan.
But Pakistan's army chief General Ashfaq Kayani said on Thursday Pakistan would defend its sovereignity and would not allow foreign troops on its soil.
Here are some facts about the controversy:
Why americans conduct cross-border attacks
Violence in Afghanistan has risen sharply over the past two years as al-Qaeda and Taliban fighters have regrouped in remote enclaves in Pakistan's ethnic Pashtun lands on the Afghan border. The United States and Afghanistan have for years pressed Pakistan to do more to eliminate the sanctuaries and stop cross-border infiltration. But Pakistani efforts, which have included military offensives and peace deals, have largely failed. Pakistani analysts say US strikes in Pakistan are also aimed at scoring points before a US presidential election in November.
Does Pakistan allow US trikes on its soil?
Officially, Pakistan forbids all military action by foreign troops on its soil but under former president Pervez Musharraf, Pakistan gave permission for missile strikes, a Pakistani official said, on the understanding the United States would inform Pakistan in advance. There has been no indication of any Pakistani permission for incursions by ground troops.
The New York Times said on Thursday US President George W Bush in July approved orders allowing ground attacks inside Pakistan without Islamabad's approval. US officials declined to comment and Pakistan's US ambassador Husain Haqqani told Reuters that Bush had not issued new orders.
US officials told the New York Times the decision illustrated lingering distrust of the Pakistani military and intelligence agencies and a belief some US operations had been compromised after Pakistanis were given details. Defence Minister Ahmed Mukhtar said in July US officials believed some Pakistani agents had tipped off militants about impending US missile attacks.
Risks for the United States and Pakistan
Support for the US-led campaign against militancy is deeply unpopular in Pakistan and more US incursions are likely to complicate problems for Pakistan's new civilian president, Asif Ali Zardari. Like his predecessor, Musharraf, Zardari is a close US ally but as an elected civilian leader he has to pay more heed to public opinion than former army chief Musharraf.
Civilian deaths in US strikes fuel anti-US sentiment, both in the immediate region and in Pakistan in general, which could undermine support for the government, led by Zardari's party. Anger over USstrikes could also increase support for religious and conservative parties that take a strong stand against the United States.
The Pakistani military fears US strikes could also fuel support for militants and even spark an uprising among Pashtuns, perhaps reviving efforts by Pashtuns to break away from Pakistan.
A breakdown in US ties would complicate economic problems because Pakistan needs US financial assistance. Pakistani foreign reserves have fallen sharply, raising fears it could default on a sovereign bond unless it gets foreign financing.