DUBAI, June 6 (Reuters) - An al Qaeda leader threatened U.S.-led forces and their Afghan allies with a deadly summer of attacks in revenge for the killing of Taliban leader Mullah Dadullah.

"O worshippers of the cross and their apostate lackeys ! I bring you good tidings -- a summer like a sword that would split your heads open," Abu Yahya al-Libi said in a video statement posted on the Internet on Wednesday.

Dadullah's killing in May by U.S. forces was seen as the most serious blow to the Taliban insurgents since their government was removed from power in 2001 by a U.S.-led coalition for harbouring bin Laden and Qaeda militants.

"The soldiers of the Afghanistan Islamic state are well prepared with martyrdom seekers and other (tactics) to seek revenge ..." Libi said.

Libi, believed to have escaped from a U.S. jail in Afghanistan in 2005, described Dadullah as a "lion of war".

Dadullah was the main architect of rising attacks, including suicide raids, against Afghan and Western troops in southern Afghanistan, as well as kidnappings of foreigners and locals and a series of beheadings.

He has been replaced by his relatively unknown brother, Mullah Bakht Mohammad.

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