International police body Interpol has issued a wanted persons notice for Briton Samantha Lewthwaite, at Kenya’s request.
Ms Lewthwaite, 29, is the widow of one of the four suicide bombers who attacked London on 7 July 2005.
Known colloquially as the “white widow”, she has been linked with Somali militant Islamist group al-Shabab.
The group was behind the attack on the Westgate shopping complex in the Kenyan capital, Nairobi, at the weekend.
The assault and subsequent four-day siege left at least 67 people dead.
An Interpol statement said she was “wanted by Kenya on charges of being in possession of explosives and conspiracy to commit a felony dating back to December 2011”.
The Interpol alert, known as a “Red Notice”, requires member countries to detain the suspect pending extradition procedures.
Ms Lewthwaite – who is believed to use the alias “Natalie Webb” – had previously only been wanted for the alleged possession of a fraudulently obtained South African passport.
She is the widow of Germaine Lindsay, one of the four bombers involved in the 7 July terror attacks in London in 2005 in which 52 people were killed and hundreds more injured.
Interpol did not link the warrant to the Nairobi attack, but it comes after much speculation linking Ms Lewthwaite to events at the Westgate complex.
Kenya is continuing three days of official mourning for the civilian and military victims of the siege.
The funeral of pregnant television and radio star Ruhila Adatia-Sood was one of many being held on Thursday.
Flags are flying at half mast amid visibly tighter security around the Kenyan capital. Security guards were scanning passengers with metal detectors before they boarded buses.
On Thursday the UK Foreign Office said that the number of British nationals known to have been killed in the attack was lower than previously thought. It had reported that six Britons were among those killed, but it now says that one of the dead previously thought to be British is in fact a Kenyan national.
But at the same time the Foreign Office has warned that the number of British dead could rise before the search of the shopping centre is over.
Somali Islamist group al-Shabab has said it had carried out the attack in retaliation for Kenyan army operations in Somalia.
The militants stormed the Westgate centre on Saturday, throwing grenades and firing indiscriminately at shoppers and staff.
Twitter posts on an al-Shabab account said the group’s militants had held 137 people hostage, and claimed the hostages had died after security forces fired chemical agents to end the siege.
A government spokesman denied any chemical agents were used, and authorities called on Kenyans to ignore militant propaganda.
Al-Shabab, which is linked to al-Qaeda, has repeatedly threatened attacks on Kenyan soil if Nairobi did not pull its troops out of Somalia.
About 4,000 Kenyan troops have been serving in the south of Somalia since October 2011 as part of an African Union force supporting Somali government forces.
The group is banned as a terrorist group by both the US and the UK and is believed to have between 7,000 and 9,000 fighters.
Its members are fighting to create an Islamic state in Somalia.