A US drone strike targeting two militant vehicles early Sunday killed at least four rebels in a restive Pakistani tribal region near the Afghan border, security officials said.
It was the second missile attack in hours in Shawal district of North Waziristan region, considered a bastion of Taliban and Al-Qaeda-linked militants.
The first strike which targeted a compound had killed at least six militants in Shuwedar village in Shawal district on Saturday.
"US drones fired four missiles on two militant vehicles in the early hours of Sunday, killing four militants," a Pakistani security official told AFP on condition of anonymity.
He said there were up to five drones flying in the area at the time of the attack.
Another security official confirmed the attack and casualties but said the identities of those killed in the strike were not immediately known.
A local intelligence official in Miranshah, however, put the toll in the strike at six.
It was the fourth drone attack since the start of the Muslim fasting month of Ramadan and the second since Pakistan's spymaster, Lieutenant General Zaheer ul-Islam, visited Washington earlier this month.
Islam's talks with his CIA counterpart were also said to have focused on drone strikes.
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 the attacks are too important to give up.
The latest attacks were in the same region where a drone strike on June 4 killed 15 militants, including senior Al-Qaeda figure Abu Yahya al-Libi.
In protest at US drone attacks, local Taliban and Pakistani warlord Hafiz Gul Bahadur have banned vaccinations in North and South Waziristan, putting 240,000 children in the region at risk.
They have condemned the immunisation campaign as a cover for espionage. In May, a Pakistani doctor was jailed for 33 years after helping the CIA find Osama bin Laden using a hepatitis vaccination programme as cover.
Washington considers Pakistan's semi-autonomous northwestern tribal belt the main hub of Taliban and Al-Qaeda militants plotting attacks on the West and in Afghanistan.
The 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 a "veritable arm" of Pakistani intelligence.
Pakistan has in turn demanded that Afghan and US forces do more to stop Pakistani Taliban crossing the Afghan border to relaunch attacks on its forces.
There has been a dramatic increase in US drone strikes in Pakistan since May, when a NATO summit in Chicago could not strike a deal to end a six-month blockade on convoys transporting supplies to coalition forces in Afghanistan.
On July 3 however, Islamabad agreed to end the blockade after the United States apologised for the deaths of 24 Pakistani soldiers in botched air strikes last November.