MIRANSHAH  - A US drone strike on Tuesday killed at least three militants in North Waziristan Agency, near the Afghan border, security officials said.Two US drones fired four missiles on a vehicle in Shnakhura village, 10 kilometres from Miranshah, the main town of North Waziristan."We have received confirmed reports that three militants have been killed in the drone strike on a vehicle. However, the identities of the militants are not yet clear," a senior security official told AFP in Peshawar.Another official in Miranshah said a compound near the vehicle was "badly damaged and engulfed in flames" in the strike. Separate but unconfirmed reports suggested more than three militants may have been killed."We suspect that the dead militants could be up to six. There are also reports that two were injured," a security official in Miranshah told AFP on condition of anonymity because he was not authorised to speak to the media.Attacks by unmanned American aircraft are deeply unpopular in Pakistan, which says they violate its sovereignty and fan anti-US sentiment, but US officials are said to believe they are too important a weapon to give up.The alleged presence of al-Qaeda-linked Haqqani network in North Waziristan, blamed for some of the deadliest attacks in Afghanistan, is one of the thorniest issues between Islamabad and Washington. Washington has long demanded that Pakistan take action against the Haqqanis, whom the United States accused of attacking the US embassy in Kabul last September and acting like the "veritable arm" of Pakistani intelligence.Pakistan has in turn demanded that Afghan and US forces do more to stop Pakistani Taliban crossing the border from Afghanistan to launch attacks on its forces. Pakistan's foreign ministry strongly protested against the drone strikes last week, saying in a statement that Islamabad "has consistently maintained that these attacks are a violation of its sovereignty and territorial integrity".There has been a dramatic increase in US drone strikes in Pakistan since May, when a Nato summit in Chicago failed to strike a deal on supplies to Afghanistan.