American-fired missiles killed 20 Islamist militants in northwest Pakistan on Wednesday, most of them members of a powerful insurgent network fighting the U.S. presence in Afghanistan, Pakistani intelligence officials said.

Two missiles slammed into a house close to the town of Miran Shah in North Waziristan, a militant hotspot that lies just across the border from Afghanistan, the officials said on condition of anonymity because they were not authorized to speak to the media.

They said 14 of the dead were Afghan militants belonging to the Haqqani network, a Taliban-linked militant faction fighting the U.S. in Afghanistan.

Six were Pakistani militants supporting the group, which America regards as one of its deadliest foes in Afghanistan, they said.

It was not possible to independently confirm the officials' account of the attack because the region is too dangerous for independent reporting.

Locals and rights groups say civilians are regularly killed in the drone strikes. There are never public investigations into those claims.

Washington began the missile program that targets Al-Qaeda and the Taliban on the Pakistani side of the border in 2005, but stepped up the pace in 2008 and again when the Obama administration came into office. At peak times, there can be as many as three or four strikes per week.

U.S. officials do not publicly talk about the covert, CIA-run program, but privately say it is crucial to keeping Al-Qaeda under pressure in one of its main international sanctuaries, as well weakening insurgent factions in Afghanistan.

But the program is a source of tension between the U.S. and Pakistan, which protests the strikes, saying they fuel militancy in the country. Over the last six months, ties between the two nations have grown increasingly strained, complicating U.S. goals for withdrawing from Afghanistan.