By Saeed Ali Achakzai

SPIN BOLDAK, Afghanistan, May 14 (Reuters) - Taliban sources confirmed on Monday that the movement's top operational commander was killed in a clash with Western and Afghan forces, and said Taliban leader Mullah Mohammad Omar was pondering a replacement.

Mullah Dadullah's killing at the weekend was widely regarded as the biggest blow to the Taliban since they began an insurgency after U.S.-led coalition troops overthrew their radical Islamic government in 2001.

Nicknamed Afghanistan's Al Zarqawi after the slain al Qaeda leader in Iraq, Dadullah was the main architect of suicide bombings, kidnapping of foreigners and Afghans, a series of beheadings and the rise of violence in the south.

"Yes, Mullah Dadullah has been killed," a Taliban source said.

"It is a very big loss for us because Dadullah was a very important commander and Taliban's war planner against coalition and Afghan forces," a senior Taliban commander, who preferred to remain anonymous, told Reuters.

Mullah Omar, whose whereabouts are not known since the Taliban's ouster, was consulting with the Taliban leadership council on Dadullah's replacement.

Dadullah was killed in a clash as part of a widespread U.S.-led operation in southern Afghanistan's Helmand province, the main bastion for the militants and the key drug producing region of the country, the world's leading heroin producer.

Authorities made Dadullah's death public only after forensic tests confirmed his identity because past reports of the notorious commander's demise turned out to be erroneous.

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