U.S. intelligence officials say al-Qaida in South Asia is now significantly weakened -- its leadership either dead or on the run. Unmanned aircraft, or drones, are partly responsible. But questions about their continued success, and even their legality, complicate the story.

Al-Qaida remains a much feared terrorist organization.

Its ties stretch across continents.

Its followers strike without warning.

But al-Qaida's traditional base of power in Pakistan and Afghanistan is greatly diminished, particularly since the killing of Osama bin Laden last May.

"As long as we sustain the pressure on it, we judge that core al-Qaida will be of largely symbolic importance to the global jihadist movement," said U.S. National Intelligence Director James Clapper.

One source of pressure comes from unmanned aircraft.  

President Barack Obama recently offered a rare acknowledgment.

"Obviously, a lot of these strikes have been in the FATA [Pakistan's Federally Administered Tribal Areas] and going after al-Qaida suspects who are up in very tough terrain along the border between Afghanistan and Pakistan.  For us to be able to get them in another way would involve probably a lot more intrusive military actions that the one that we're already engaging in," Mr. Obama said.

Mr. Obama spoke about drones in a Web chat. He denied the strikes cause high civilian casualties.  "For the most part they have been very precise, precision strikes against al-Qaida and their affiliates, and we are very careful in terms of how it's been applied," the president said.

Defense specialist Thomas Lynch says intelligence operations and drone strikes have significantly diminished al-Qaida's presence in the Afghanistan-Pakistan border region.  But he says drones may not be able to target the remaining terrorist leaders there.

"Those characters are not likely to be out and vulnerable to drone strikes anymore right now anyway.  I mean, they've learned their lesson.  They are much more likely to be in and around large urban areas where we're going to need Pakistani support and assistance to get at them anyway," Lynch said.

Counterterrorism operations can raise concerns.  "The real issue is, should the United States be allowed to fly airplanes over a sovereign country and kill people on the ground?," said Tom Parker with Amnesty International in Washington.

Also, says Parker, the U.S. government has not explained if strikes are being conducted in the name of self defense or as part of a global war against a terror threat.  

"An international armed conflict globally means the United States is claiming the right to kill people it thinks are terrorists anywhere in the world.  Now that does include Pakistan, that does include Yemen, but conceptually at least, it also includes Mexico, Paris, Liberia, South Africa and Guatemala, right, and for that matter the streets of the United States itself.  Now, that's alarming," Parker said.      

Rights groups are urging President Obama or Attorney General Eric Holder to offer more clarity about the scope of covert operations.