A U.S. drone aircraft fired missiles in Pakistan's South Waziristan region on the Afghan border, killing at least 15 militants on Sunday, Pakistani security officials said.
The United States has stepped up pilotless drone strikes in Pakistan's tribal regions since a Dec. 30 suicide bombing killed seven CIA employees at a U.S. base in neighboring Afghanistan.
"Now the death toll is 20. It could rise further. People are still busy removing rubble," a senior security official told Reuters. Most of the casualties were foreign fighters, he added.
Ten days after the Afghanistan attack by a Jordanian double agent -- the second bloodiest in the agency's history -- Pakistani Taliban leader Hakimullah Mehsud appeared sitting beside the suicide bomber in a farewell video.
The bomber said he would avenge the death of a Taliban leader killed in a drone strike last year and called for attacks in and outside the United States.
The clip raised Mehsud's profile and suggested his group -- which has been fighting the Pakistani state -- was now a greater force to be reckoned with and had forged closer ties with al-Qaeda, which claimed responsibility for the attack.
Pakistani Taliban militants issued an audio tape on Saturday purportedly from Mehsud, denying he had been killed in a U.S. missile strike two days earlier.
The pilotless drone strikes are a source of friction between the United States and Pakistan, which says they are a violation of its sovereignty. U.S. officials say the missile strikes are carried out under an agreement with Islamabad that allows Pakistani leaders to decry the attacks in public. Islamabad denies this.
Washington says they are an effective weapon in tribal areas in northwest Pakistan along the Afghan border seen as a global hub for militants, including senior al-Qaeda and Taliban figures.
Pakistan has resisted U.S. pressure to tackle Afghan militant groups who use its lawless tribal areas as a rear base for fighting Western troops in Afghanistan, saying they are focused on homegrown Taliban enemies.
Sunday's attack was the tenth missile strike by an unmanned U.S. spy plane so far this year, as the administration of U.S. President Barack Obama puts Pakistan at the heart of its fight against al-Qaeda and Islamist extremists.
The latest attack also brought to 41 the number of militants reported to have been killed in three attacks since Thursday.