Adan Garar and two others are suspected to have been killed after their car was targeted near the town of Bardhere, the Kenyan and US officials said.
The senior Kenyan official, who insisted on anonymity because he wasn't authorised to give the information to the media, says Garar is also suspected of planning failed attacks on Kenya's coast and in the Ugandan capital, Kampala, last year. The US official also insisted on anonymity.
Al-Shabab, an Islamic extremist group, has vowed to inflict violent attacks on Kenya and Uganda because the two countries have contributed troops to the African Union force supporting the government in Somalia.
Sixty-seven people were killed in the Westgate attack, which was carried out by four gunmen from al-Shabab.