It is feared the 29-year-old could be on her way to Yemen where she has spent time with leading members of Al-Qaida.
Detective searching for White Widow Samantha Lewthwaite are probing reports she is hiding out in a remote area of Somalia.
Acting on a tip-off, officers are searching in Baidoa for the fugitive believed to have played a key role in the Nairobi mall massacre.
A security source said: “The sighting is believed to be one of the most credible intelligence reports received in a long time concerning where Samantha is.”
It is feared the 29-year-old, originally from Aylesbury, Buckinghamshire, could be on her way to Yemen where she has spent time with leading members of Al-Qaida.
The alleged sighting follows the Daily Mirror exclusive that Lewthwaite is believed to be behind seven unsolved murders in Kenya.
She is said to have ordered the murders of four religious leaders and three others linked to her al-Shabaab terror group.
Anti-terror police named the White Widow as number one suspect in the cowardly slaughter of Protestant pastors Charles Matole and Ebrahim Kidata.
They also believe she was behind the killings of radical Muslim clerics Aboud Rogo Mohammed and his successor Sheik Ibrahim Rogo after falling out with them.
Mr Matole, 41, was shot in the head at his church in Mombasa. He was found on October 19 sitting in a chair still clutching his bible.
Security sources believe he was executed after mum-of-four Lewthwaite became “paranoid” over his contacts with Kenyan intelligence officials.
Security services also believe the Nairobi mall massacre suspect has infuriated elders within al-Shabaab after demanding more control over its murderous activities and is locked in a power struggle.
Seven of Lewthwaite’s disciples, including a Tanzanian preacher, a Somali national, a Ugandan and three Kenyans, are also being hunted.
Ahead of the Nairobi atrocity she was already wanted by Kenyan police over links to a terrorist cell planning bombs attacks.
The widow of 7/7 Kings Cross bomber Germaine Lindsay, Lewthwaite is also accused of planning a bomb attack to kill hundreds of British tourists in Mombasa with fellow British accomplice Jermaine Grant.
See on www.mirror.co.uk