It termed Iswap and its affiliates in the region as the vanguard of the Islamic State.
The Nigerian government has repeatedly claimed that the terror groups have been technically defeated.
But the head of the US Counter-terrorism Bureau, Nathan Sales, told the BBC that since the loss of the Islamic State group’s caliphate in Iraq and Syria, its affiliates in West Africa have become crucial to its survival.
He said the US would contribute over $30m (£22.6m) in counter-terrorism support to the region over the next year.
Iswap has carried out a number of recent attacks against Nigerian authorities, including multiple ambushes on convoys belonging to the governor of the northern state of Borno.