Afghan President Hamid Karzai says the death of Abu Musab al-Zarqawi, the al-Qaeda leader in Iraq, is a "severe blow" to global terrorism.
Zarqawi urged militants to fight the US in Afghanistan
Mr Karzai said Zarqawi had been responsible for the deaths of thousands of Muslims in Iraq and Afghanistan.
The US military says Zarqawi was killed in an air raid north of the town of Baquba on Wednesday evening.
Zarqawi spent time with the Taleban in Afghanistan and from Iraq supported their fight against US-led forces.
Zarqawi's death is "a significant step in ridding the world of the menace of terrorism," Mr Karzai said in a statement issued on Thursday.
"This success should encourage us all, both in the Muslim world and beyond, to continue and step up our common fight against terrorism."
Zarqawi spent some time in Afghanistan fighting against Soviet forces.
He returned to Afghanistan for a longer spell when the Taleban were in control of the country. He left after the US-led invasion in 2001.
"Zarqawi and his terrorist organisation were responsible for the killing of thousands of Muslims in Iraq and in Afghanistan," Mr Karzai said.
Zarqawi continued to play a role offering support to militants in Afghanistan fighting US and other foreign forces in the country.
Correspondents say it is not clear what impact news of Zarqawi's death will have on his allies in Afghanistan.