The US military says a reward of up to $10m is still available for information leading to the capture of key ex-Saddam Hussein aide Izzat Ibrahim al-Douri.
Douri is believed to have helped finance the insurgency
They are treating with caution claims on websites linked to the former ruling Baath party that Douri died of cancer.
Some Baathist websites which published the original death notice also appear to be questioning the announcement.
One carried a statement from loyal Saddam Hussein aide Salah al-Mukhtar saying Douri had not died.
"To my brothers and sisters, I want to tell you that [Saturday's] statement was false and Mr Izzat Ibrahim al-Douri is in good health," said the statement, signed by Mr Mukhtar, who was Saddam Hussein's former ambassador to India.
Another website said the Baath party communique had been forged by what it called the occupation authority's intelligence and their local hirelings in order to sow confusion and chaos among the ranks of the resistance.
The death notice disappeared abruptly from its web pages after standing for 24 hours, the BBC's Jim Muir in Baghdad says.
In other news:
- Leaders of the minority Sunni Arabs in Iraq warn that military and police crackdowns are alienating their community ahead of December's parliamentary elections
- More than 350 people - including five local officials - have been arrested by Iraqi police commandos in the restive town of Baquba on Saturday.
Douri, 63, was the second most powerful man in Saddam Hussein's Iraq and is regarded by the US as their most wanted man after the leader of al-Qaeda in Iraq, Abu Musab al-Zarqawi.
"A reward of up to $10m remains for information leading to al-Douri's capture or his gravesite," the US military said on Sunday.
But the US also said evidence showed Douri was losing influence within the Baath party "since he is unable to maintain the visibility and contacts required to uphold loyalty among what little remains of his network and supporters".
Douri was number six on the US list of 55 most-wanted members of Saddam Hussein's regime. The top five have all been captured.
He was diagnosed with leukaemia several years ago - but his subsequent recovery enabled him to pursue a challenging domestic and foreign work schedule.
He often represented Iraq at foreign gatherings - most markedly in the run-up to the US-led invasion of April 2003.
Douri was considered to have been Saddam Hussein's daily right-hand man.
He was deputy commander-in-chief of the armed forces and held a senior post on the committee responsible for northern Iraq when chemical weapons were used in 1988, killing thousands of Kurds at Halabja.
He served as vice-chairman of the Revolutionary Command Council and was a key commander in the suppression of the failed Shia uprising in 1991.
There have been persistent reports that he was unwell ever since he went into hiding following the invasion.
In recent years, he has been accused of financing insurgent groups in Iraq.