ISLAMABAD (Reuters) -- Violence has rocked nuclear-armed Pakistan, with a pilotless U.S. drone aircraft killing 13 people including militants in the northwest and a suicide bomber killing eight soldiers in the capital, Islamabad.
Pakistan is crucial to U.S. efforts to stabilise neighbouring Afghanistan. U.S. President Barack Obama said the release of additional U.S. aid to Pakistan would depend on how it tackled terrorism.
With the Afghan insurgency intensifying, the United States began launching more drone strikes against Al-Qaeda and Taliban militants on the Pakistani side of the border last year.
Since then, more than 30 U.S. strikes have killed about 350 people, including mid-level Al-Qaeda members, according to reports from Pakistani officials, residents, and militants.
Pakistan calls the strikes violations of its sovereignty and says the civilian casualties they inevitably cause inflame anti-U.S. sentiment, complicating its effort to fight militancy.
The attack on April 4 was in North Waziristan, a stronghold of Al-Qaeda and Taliban militants on the Afghan border about 35 kilometers west of the region's main town of Miranshah.
Hours later, a lone suicide bomber killed eight paramilitary troops in an attack on their post in the heart of Islamabad, a city official said. Five were wounded.
Pakistani Taliban militants have threatened in recent days to launch attacks in the capital, in Afghanistan, and in the United States in retaliation for the drone attacks.
Pakistani Taliban leader Baitullah Mehsud claimed on April 4 responsibility for a shooting at a U.S. immigration center in New York state in which a gunman killed 13 people. Mehsud said it was revenge for the drone attacks.
But U.S. officials ruled out that claim, and Pakistani security analysts dismissed it as a publicity stunt.
Hours after the drone strike, a suicide bomber was killed as he approached a military convoy near Miranshah. His explosives went off, killing three passersby, witnesses and a hospital official said.
Many Al-Qaeda and Taliban militants fled to North Waziristan and other northwestern Pakistani border regions after U.S.-led forces ousted the Taliban in Afghanistan in 2001.
From the remote ethnic Pashtun tribal lands that have never been truly ruled by any Pakistani government, militants have orchestrated the Afghan war and plotted violence beyond.
U.S. commanders say the militant enclaves have to be eliminated, and Obama has said the United States will tackle them if Pakistan will not or cannot.
Pakistani officials say civilian deaths in drone strikes fuel anti-U.S. sentiment, complicating the military's struggle to subdue violence. The concentration of strikes in Waziristan is also pushing militants deeper into Pakistan, they say.
Obama, speaking at the end of a NATO summit in France, said Pakistan had to have the capacity to tackle Al-Qaeda.
"I informed our allies that despite difficult circumstances we are going to put more money into Pakistan, conditional on action to meet the terrorist threat," he told a news conference. "We want to bring all of our diplomatic and development skills to bear towards strengthening Pakistan, in part because they have to have the capacity to take on Al-Qaeda within their borders."
President Asif Ali Zardari, the widower of former prime minister Benazir Bhutto, and his year-old civilian government are also struggling to revive a flagging economy.
The U.S. special envoy to Afghanistan and Pakistan, Richard Holbrooke, is due in Pakistan in coming days.