WASHINGTON — The operational leader for Al Qaeda in Afghanistan was killed in an American missile strike in Pakistan’s tribal areas in the last two weeks, according to a statement the group issued late Monday that American officials believe is correct.

The militant leader, Mustafa Abu al-Yazid, an Egyptian, was a top financial chief for Al Qaeda as well as one of the group’s founders, and was considered by American intelligence officials to be the organization’s No. 3 leader, behind Osama bin Laden and his deputy, Ayman al-Zawahri, another Egyptian.

“His death will only be a severe curse by his life upon the infidels,” Al Qaeda said in a statement issued to jihadist Web sites and translated by the SITE Intelligence Group, which monitors statements by jihadists.

A United States official said American intelligence analysts believed the statement from Al Qaeda was accurate. They said the death of Mr. Yazid, also known as Saeed al-Masri, was a significant setback to Al Qaeda’s ability to help plan and support cross-border attacks against American and allied forces in Afghanistan from its haven in Pakistan’s tribal areas.