A massive US airstrike in Yemen has killed what the Pentagon estimates is “dozens” of people, the second such mass-casualty strike the US military has undertaken this month.

The two strikes, killing more than 200 people at what the Pentagon described as terrorist training camps, diverged so sharply from the previous years’ worth of relatively low-casualty strikes that observers speculated US policy may have quietly changed.

Peter Cook, the Pentagon spokesman, announced late Tuesday that the US had bombed a mountain redoubt in Yemen used by al-Qaida’s local affiliate, al-Qaida in the Arabian Peninsula (AQAP). He said it was a “training camp” used by “more than 70 AQAP terrorists”.

An independent assessment of the actual impact of the strike, to include a full casualty total and civilian impact, was not immediately available. The Pentagon did not provide further detail of where in Yemen the alleged camp was located.

“We continue to assess the results of the operation, but our initial assessment is that dozens of AQAP fighters have been removed from the battlefield,” Cook said in a statement.

The US airstrike happened hours after multiple attacks killed over 30 people and wounded more than 200 in Brussels, although it is unclear if any connection exists between the two events. Islamic State has claimed credit for the Brussels attacks.

On 5 March, a different US airstrike killed an estimated 150 people in Somalia, also targeting what the Pentagon described as a terrorist training camp, this one used by al-Shabaab. The US military predicated the strike, which used MQ-9 Reaper drones as well as piloted warplanes, on preventing al-Shabaab from attacking US-aligned African forces in Somalia. An independent witness contacted by the Guardian described al-Shabaab fighters “collecting dead bodies”.

While all counter-terrorism airstrikes during Barack Obama’s presidency have occurred under a veil of official secrecy, years’ worth of outside analysis has suggested that the strikes typically kill fewer than a dozen fighters at once – either by design or due to the relatively small Hellfire missile carried by US drones. Rarely does the Pentagon announce the targeting of training camps or other large gatherings.

Micah Zenko, a counter-terrorism analyst at the Council on Foreign Relations who tracks the strikes, estimated that the US has carried out 575 airstrikes in Yemen, Somalia and Pakistan, killing around 4,000 people, both militants and civilians. The casualty toll from those earlier strikes suggests an average of seven deaths per strike.

Zenko speculated that the two most recent strikes in Somalia and Yemen resembled conventional-war airstrikes more than they do “targeted killings”, the White House’s preferred term for its lethal counter-terrorism measures.

“The Somalia and Yemen strikes suggest that the White House has authorized a significant opening of the aperture to target gatherings of suspected terror groups, rather than named individuals who pose imminent threats,” Zenko said.

The White House deferred comment to the Pentagon, which denied any policy shift. “This strike was conducted consistent with the policy for counter-terrorism direct action announced by the president in May 2013,” spokesman Maj Ben Sakrisson said.

Cook, the Pentagon spokesman, declared in his initial statement that the Yemen strike “demonstrates our commitment to defeating al-Qaida and denying it safe haven”.