The United States is planning to send about 200 troops to Nigeria to train the African nation's military to fight Islamist militants, a US official said on Tuesday, weeks after President Donald Trump ordered airstrikes against what he called Islamic State targets.

The US military said last week it sent a small team of troops to Nigeria without specifying a number, marking the first acknowledgement of US forces on the ground since Washington struck by air on Christmas Day.

Trump has said there could be more US military action in Nigeria and Reuters has reported that the US had been conducting surveillance flights over the country from Ghana since at least late November.

The official said the 200 troops will supplement a handful of US military personnel already in Nigeria to help local forces.

US Africa Command did not immediately respond to a request for comment.

Nigeria has come under intense pressure from Washington to act after Trump alleged the West African nation was failing to protect Christians from Islamist militants operating in the northwest.

The Nigerian government denies any systematic persecution of Christians, saying it is targeting Islamist fighters and other armed groups that attack and kill both Christians and Muslims.

Boko Haram and Islamic State West Africa Province (ISWAP) fighters have intensified attacks on military convoys and civilians, and the northwest remains an epicenter of a 17-year Islamist insurgency.

Nigeria's population of over 230-million people is roughly evenly divided among Christians, who predominate in the south, and Muslims, who predominate in the north.