As the United States pressed ahead with a grinding campaign in Afghanistan in 2010, President Barack Obama dramatically escalated another war across the border in Pakistan, using robotic planes to pound Al-Qaeda and Taliban militants.

The effect of the expanding covert war remains unclear and some skeptics have warned civilian casualties from the strikes could ultimately feed extremism in Pakistan's tribal areas. But US officials say Al-Qaeda's leadership has been severely weakened.

Coinciding with an influx of US troops in the Afghan war, Obama has pursued the "surge" in CIA bombing raids in Pakistan's northwest, despite criticism from rights groups that the strikes amount to extrajudicial killings.

As of December 17, Predator and Reaper drones armed with Hellfire missiles and precision-guided bombs carried out 113 strikes against Islamist militants in Pakistan, double the number in 2009 and more than the total number of raids conducted in the previous six years, according to a tally by the independent New America Foundation.

The covert bombing raids are backed up by a clandestine CIA-run paramilitary force of 3,000 Afghans, reportedly carrying out sensitive cross-border operations in Pakistan.

Unlike the nine-year-old conflict in Afghanistan, the drone war has steadily expanded with little US public debate while American officials avoid openly discussing the CIA raids.

"By the old standards, this would be viewed as a war," Peter Singer, author of a book on robotic weapons, "Wired for War," told a congressional hearing in March.

"But why do we not view it as such? Is it because it is being run by the CIA, not by the military and thus not following the same lines of authority and authorization?" he asked.

"Is it because Congress never debated it?"

Officials credit the drone strikes with knocking out hundreds of insurgents, including some senior figures, with media reports putting the toll as high as 897 militants.

But the number of civilian casualties caused by the strikes in the remote tribal areas is hotly disputed and virtually impossible to verify.

US officials have privately maintained that civilians only make up one or two percent of those killed by the strikes, while others charge the raids have caused hundreds casualties.

The strikes reflect Washington's uneasy alliance with Pakistan, which tacitly cooperates with the bombing raids but has been reluctant to go after the Haqqani network and other militants in North Waziristan and elsewhere.

The drone raids, and the civilian casualties associated with them, are deeply unpopular in Pakistan, placing the Islamabad government in an uncomfortable position.

The overwhelming majority of the raids are carried out in North Waziristan, and US officials have pressed Islamabad for permission to expand the drone war to other areas, according to the new book "Obama's War" by the Washington Post's Bob Woodward.

With Al-Qaeda's affiliates in Yemen and elsewhere posing a mounting threat, the remotely-piloted drones have become the weapon of choice for US strikes against terror suspects.

Recently leaked diplomatic cables confirmed the US mounted missile strikes against Al-Qaeda in Yemen, even though the country's leaders publicly claimed their own forces carried out the attack.

Critics question the tactic, saying militant networks are able to replace fallen leaders and that countering Islamist extremists requires prevailing in a broader struggle for the hearts and minds of Muslims.

The drone raids are akin to "going after a beehive, one bee at a time," former CIA officer Bruce Reidel told the New Yorker last year, and "the hive will always produce more bees."

But there are no other good options, he said.

"It's really all we've got to disrupt Al-Qaeda. The reason the administration continues to use it is obvious: it doesn't really have anything else."