Mr Gates visited US forces and local leaders in Afghanistan on Wednesday
US Defence Secretary Robert Gates has defended the right of American forces to strike at militants on the Pakistani side of the Afghan border.
Asked by the BBC if Pakistan had authorised such air strikes, he said that the US would take "whatever actions necessary" in self-defence.
Washington would prefer for Pakistan to tackle militants itself, he added.
Mr Gates, fresh from a visit to Afghanistan, is in London for talks with other Nato ministers.
Earlier on Thursday, he said that increasing militant violence there might prompt a change in US strategy, but gave no details.
Last week, America's top general called for a new strategy to cover both sides of the Afghanistan-Pakistan border.
Recent cross-border US raids on militants have been condemned by Pakistan's new government, which insists it will not allow foreign forces on to its territory.
Speaking to BBC security correspondent Frank Gardner, Mr Gates welcomed the Pakistani army's "much more aggressive" approach to militants along the border in recent weeks.
Robert Gates speaks to the BBC
He said the militants were the "common enemy" of Pakistan, Afghanistan, and US troops and their allies.
"Our goal is to partner with the Pakistanis and enable them to deal with this threat on the border themselves," he said.
Had the Pakistani government authorised US air strikes on Pakistani territory, our correspondent asked.
"I wouldn't go in that direction," Mr Gates replied.
"I would just say that we will take whatever action necessary to protect our troops."
Speaking to reporters earlier, Mr Gates said that strategy in Afghanistan had to be adjusted "continually, based on the circumstances that you find".
"We did that in Iraq," he added. "We made a change in strategy in Iraq and we are going to continue to look at the situation in Afghanistan."
Addressing a Congressional committee last Thursday, the chairman of the US Joint Chiefs of Staff, Adm Mike Mullen, called for a new strategy in Afghanistan to deny militants bases across the border in Pakistan.
The US must work closely with Pakistan to "eliminate [the enemy's] safe havens", he said.
The strategy for Afghanistan is a priority for Gen David Petraeus, due to oversee US military operations throughout the Middle East, Afghanistan and Pakistan, as head of Central Command from the end of October.
Speaking recently to the BBC, the former US commander in Iraq said he had to turn around the Nato mission in Afghanistan, where the trend was "in the wrong direction".
Mr Gates told the BBC that the US tried to keep civilian casualties to a minimum. He accused the Taleban of using them as human shields.
The issue has caused increasing anger in Afghanistan.
US forces have promised to review an inquiry into an air raid last month in the province of Herat which was blamed for the deaths of scores of civilians.
Figures released by the UN on Tuesday showed that there had been a sharp increase in the number of civilian casualties in Afghanistan this year.
About 200 foreign soldiers have been killed in Afghanistan so far this year - about the same number as died during the whole of last year.
A bomb attack killed four US soldiers and an Afghan national in the east of the country on Wednesday.
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