A surprise attack by U.S. drones on an Islamist training camp in Somalia Sunday killed more than 150 members of the al-Shabaab militant group, including five alleged top commanders, according to various reports.

The al-Qaeda linked group reportedly confirmed the airstrike but instantly rejected the death toll as exaggerated, saying the U.S. usually "exaggerate when they do something against us.''

Senior defence officials told NBC News that the fatalities were among the 200 or so fighters at the training camp, some 120 miles north of Mogadishu who were "preparing for an imminent large-scale attack against U.S. and regional governmental forces.''

"It appeared the group was in formation and conducting some kind of ceremony when American drones dropped bombs and launched Hellfire missiles at the militants,'' the officials said, according to NBC News.

Abdiasis Mohamed Durow, district commissioner in Buloburte said the death toll could even be higher and that some top commanders of the group were killed.

"I have confirmed that this was a heavy and deadly bombardment. This time al-Shabaab lost about 200 fighters, among them five top commanders who were at the training camp to witness the closing of the training of the newly recruited militants,'' he said, the Guardian reported.

Two of the commanders who were killed at the camp, he added, were Yusuf Al Ugaas, an influential preacher, recruiter and regional head; and Mohamed Mire, a leading member of the group's finance wing.

The airstrike was like "a burnt house,'' according to Bashir Dhure, a cattle herder who lives near the training facility and witnessed the attack.

He told the Guardian in a telephone interview that he also saw three vehicles that went down while members of the group were collecting dead bodies, loading them on the trucks before leaving the village.

After the bomb attack, Dhure said the group started to search for alleged "spies" and detained about a dozen people, mostly young camel herders living in two nearby villages, Eel Dibi and Raso, about 125 miles north of the capital, Mogadishu.

He said he was lucky to have fled the area before the militants began search operation.

Gen. David Rodriguez of the U.S Africa Command, said they had been monitoring the camp for several weeks before the strike and had gathered intelligence, including about an imminent threat to U.S. forces and African Union peacekeepers. He added that the Raso training camp was one of several that al-Shabaab maintains in Somalia.

The Somali government welcomed the killing of the militants, said Ali Abduraham, a senior Somali intelligence officer in Hiiraan region.

"The attack on Al-Shabaab was a very important operation because we knew the militants were in their latest stage of attacking key places in Somalia.''

He added the group planned to wage deadly attacks on Mogadishu and Kismayo where the U.S. has set up a drone base.

Al-Shabaab has been responsible for terrorist attacks throughout East Africa, including the massacre of 67 civilians during a four-day siege at the Westgate Mall in Nairobi, Kenya, in September 2013, NBC News said.