Washington formally determined Tuesday that Myanmar's military carried out a coup, legally requiring an end to US assistance to the government.

"After a careful review of the facts and circumstances, we've assessed that Aung San Suu Kyi, the leader of Burma's ruling party, and Win Myint, the duly elected head of government, were deposed in a military coup," a State Department official said, using Myanmar's former name.

"We continue to call on the Burmese military leadership to release them, and all other detained civil society, and political leaders immediately and unconditionally," she said.

Under US law, the United States will be forbidden from assisting the government but the effects will be largely symbolic as virtually all assistance in Myanmar goes to non-governmental channels.

The military was already under US sanctions over its brutal campaign against the Rohingya minority.

The State Department official, speaking on customary condition of anonymity, said Washington would maintain humanitarian programs, including for the Rohingya, but will also "undertake a broader review of our assistance" to Myanmar.

President Joe Biden in a forceful statement Monday said the US would consider reimposing sanctions on Myanmar, which were lifted during its decade-long transition to democracy.

Washington has contributed $1.5 billion to Myanmar since 2012 to support democracy, internal peace and violence-hit communities, according to the State Department.