The United States has begun launching drone strikes against suspected al-Qaeda operatives in Yemen under new authority approved by President Obama that allows the CIA and the military to fire even when the identity of those who could be killed is not known, U.S. officials said.

The policy shift marks a significant expansion of the clandestine drone war against an al-Qaeda affiliate that has seized large ­pieces of territory in Yemen and is linked to a series of terrorist plots against the United States.

U.S. officials said that Obama approved the use of “signature” strikes this month and that the killing of an al-Qaeda operative near the border of Yemen’s Marib province this week was among the first attacks carried out under the new authority.

The decision to give the CIA and the U.S. Joint Special Operations Command (JSOC) greater leeway is almost certain to escalate a drone campaign that has accelerated significantly this year, with at least nine strikes in under four months. The number is about equal to the sum of airstrikes all last year.

The expanded authority will allow the CIA and JSOC to fire on targets based solely on their intelligence “signatures” — patterns of behavior that are detected through signals intercepts, human sources and aerial surveillance, and that indicate the presence of an important operative or a plot against U.S. interests.

Until now, the administration had allowed strikes only against known terrorist leaders who appear on secret CIA and JSOC target lists and whose location can be confirmed.

Moving beyond those rules of engagement raises substantial risks for the Obama administration, which has sought to avoid being drawn into a fight between insurgents and Yemen’s central government.

Congressional officials have expressed concern that using signature strikes would raise the likelihood of killing militants who are not involved in plots against the United States, angering Yemeni tribes and potentially creating a new crop of al-Qaeda recruits.

Critics have also challenged the legal grounds for expanding the drone campaign in Yemen. In an opinion piece published in The Washington Post on Sunday, Bruce Ackerman, a law professor at Yale University, argued that war measures adopted in the aftermath of the Sept. 11, 2001, attacks were not aimed at al-Qaeda’s Yemeni affiliate and don’t provide Obama “with authority to respond to these threats without seeking further congressional consent.”

The Post reported last week that the CIA was seeking authority to expand the drone campaign in Yemen. The approval of that enhanced authority was first reported Wednesday on the Wall Street Journal’s Web site.

CIA and White House officials declined to comment.

Administration officials stressed that U.S. airstrikes in Yemen will still be under tighter restrictions than they have been in Pakistan. CIA drones flying over Pakistan’s tribal belt are allowed to strike groups of armed militants traveling by truck toward the war in Afghanistan, for example, even when there is no indication of the presence of al-Qaeda operatives or a high-value terrorist.

In Yemen, by contrast, signature strikes will only be allowed when there is clear indication of the presence of an al-Qaeda leader or of plotting against targets in the United States or Americans overseas. In recent months, U.S. spy agencies have collected intelligence indicating plots against American diplomats or U.S. special operations troops who are working alongside Yemeni counter-terrorism units.

But much of the expertise that the CIA and JSOC will employ in Yemen is likely to draw heavily on the agency’s experience in Pakistan. There, officials said, the CIA has become so proficient at monitoring militant groups that it can tell when an al-Qaeda leader is present at a compound through chatter on signals intercepts, security precautions taken before the dignitary’s arrival, as well as the number and behavior of al-Qaeda security personnel around the perimeter of the site.

The target of this week’s strike was an alleged al-Qaeda operative, Mohammed Saeed al Umda, who is thought to have trained at a camp in Afghanistan before the U.S. invasion in 2001. Yemeni officials described Umda, reportedly also known as Ghareeb al Taizi, as a commander of military operations for al-Qaeda in the Arabian Peninsula, or AQAP.

But Gregory Johnsen, a Yemen expert at Princeton University, has questioned whether Umda was a high-ranking figure in the group as well as the wisdom of the expanded drone operations.

“Body bags are not a good barometer for success in a war like this,” Johnsen wrote on his blog, responding to reports that the CIA was seeking to use signature strikes. “I would argue that U.S. missile strike[s] are actually one of the major — not the only, but a major — factor in AQAP’s growing strength.

AQAP has significantly expanded in numbers, strength and territory since one of its top leaders, the U.S.-born cleric Anwar al-Awlaki, was killed in a CIA drone strike last year. White House counterterrorism adviser John Brennan described AQAP as “very, very dangerous” in a speech at New York police headquarters last week, according to an account by CNN.

AQAP has more than 1,000 members in Yemen and “close connections” to al-Qaeda leadership in Pakistan, Brennan said, according to CNN. “We are very concerned about AQAP. It’s the most active operational franchise.”

AQAP has been tied to terrorist plots including the 2010 attempt to mail parcels packed with explosives to addresses in Chicago and the attempt to blow up a Detroit-bound airliner on Christmas Day 2009.

The U.S. military has carried out airstrikes using drones as well as conventional aircraft and ship-based missiles for several years. The CIA joined the hunt last year when it opened a secret drone base at an undisclosed location on the Arabian Peninsula.