The deaths were touted by America's top general in Iraq as possibly the most significant blow to the terror group since the beginning of the insurgency and a sign of the growing strength of Iraqi security forces.

But the news, which comes as US forces prepare to end combat operations, belies the resiliency of terror groups that have shown their ability to change tactics and launch new and deadly attacks despite repeated strikes to their leadership.

Iraqi Prime Minister Nuri Al-Maliki announced the killings of Abu Omar Al-Baghdadi and Abu Ayyub Al-Masri at a news conference in Baghdad and showed reporters photographs of their bloody corpses. The deaths were later confirmed by US military officials in a statement.

The US military said they were killed in a nighttime raid on their safe house Sunday near Tikrit, the hometown of former Iraqi dictator Saddam Hussein. An American helicopter crashed during the assault, killing one US soldier, the military said.

US forces commander Gen. Raymond Odierno praised the operation.

“The death of these terrorists is potentially the most significant blow to Al-Qaeda in Iraq since the beginning of the insurgency,” he said. “There is still work to do but this is a significant step forward in ridding Iraq of terrorists.”

Al-Masri was the shadowy national leader of Al-Qaeda from Iraq, which he took over after its Jordanian-born founder, Abu Musab Al-Zarqawi, was killed in a June 2006 US airstrike.

Al-Masri's real name was Abdul-Monim Al-Badawi, according to a 2009 Al-Qaeda statement describing the makeup of a new “War Cabinet.”

Al-Baghdadi is the self-described leader of the Al-Qaeda linked Islamic State of Iraq, identified by US military officials Monday as Hamid Dawud Muhammad Khalil Al-Zawi.

Past Iraqi claims to have captured or killed Al-Baghdadi have turned out to be wrong, and the Islamic State of Iraq has issued at least two denials of his capture.

Al-Baghdadi was so elusive that at times US officials also have questioned whether he was a real person or merely a composite of a terrorist that the ISI invented to bolster its threats. The US military once even asserted that audio recordings in the name of a fictitious Al-Baghdadi were in fact read by someone else.