The US said it was hiking the bounty for information leading to the identification of Al-Shabab "emir" Ahmed Diriye, second-in-command Mahad Karate and Jehad Mostafa, a US citizen who it said had various roles in the group.
The US State Department also said it was for the first time offering a reward of up to $10 million for information "leading to the disruption of the financial mechanisms" of the Al-Qaeda affiliate.
The US statement said Diriye, who has been leader since September 2014, was designated by the State Department as a "specially designated global terrorist" in April 2015, and slapped with UN sanctions the same year.
Karate, who was also designated a terrorist in April 2015, continues to lead some Al-Shabab operations, the US said.
He also "maintains some command responsibility over Amniyat, the group's intelligence and security wing, which oversees suicide attacks and assassinations in Somalia, Kenya, and other countries in the region, and provides logistics and support for al-Shabaab's terrorist activities."
Mostafa, a US citizen who once lived in California, has been a military instructor at Al-Shabab training camps, as well as a leader of foreign fighters, a leader in the group's media wing, an intermediary with other "terrorist organizations," and a leader in the use of explosives in attacks, the US said.
In December 2019, he was indicted in a US court on various charges linked to Al-Shabab.
"The FBI assesses Mostafa to be the highest-ranking terrorist with US citizenship fighting overseas."
Al-Shabaab militants have stepped up attacks in the Somali capital Mogadishu and other parts of the country in the face of a widescale offensive against the group by the new government of President Hassan Sheikh Mohamud.
UN human rights chief Volker Turk said earlier Monday that more than 600 civilians had been killed this year in attacks by the group in the fragile Horn of Africa nation.
In the deadliest attack in five years, twin bombings on October 29 claimed by Al-Shabab killed at least 121 people and injured 333 others in Mogadishu, the UN said, citing Somali figures.
The State Department designated Al-Shabab as a foreign terrorist organization in March 2008.