Sanaa, Yemen (CNN) -- Five U.S. drone strikes killed six suspected al Qaeda militants in the southeastern Yemeni province of Shabwa on Monday, two security officials and one defense ministry official told CNN.
The drones targeted militant hideouts, checkpoints, training facilities and weapons warehouses in the Azzan districts, considered the stronghold for al Qaeda in Yemen, officials confirmed.
Al Qaeda took over Azzan district March of last year, from which it has launched numerous attacks in the south of Yemen. It was the first district to fall into the hands of Islamic militants in Yemen.
Residents told CNN that the explosions were powerful and were heard miles away. Two security officials expected the death toll to rise.
One of the strikes targeted a checkpoint run by militants near the entrance of the district.
"The attacks were fierce and we have not been able to get complete death tolls from the attacks due to the security situation in Azzan," one official not authorized to talk told CNN.
Dozens of militants blocked roads leading to the areas of the attack, residents said.
Experts are concerned that the growing number of U.S. strikes will cause a backlash and hurt the Yemen's efforts in fighting terrorism.
According to two Defense Ministry officials, at least 11 U.S. attacks were conducted on Yemeni soil over the last week alone.
"U.S. involvement is far more than ever in Yemen. We have no evidence that all those being killed are terrorists," Abdul Salam Mohammed, director of Abaad Strategic Center, told CNN.
"With every U.S. attack that is conducted in Yemen al Qaeda is only growing in power and we have to ask ourselves why that is happening."
On Monday, the Defense Ministry announced that seven al Qaeda militants were killed in Abyan province, where government forces have been clashing with militants for 11 months. Two of the killed were from Somalia, the ministry said.