About 100 U.S. military personnel have arrived in Nigeria as Washington scales up an operation to target Islamist insurgents, a Nigerian defence spokesperson said.

U.S. President Donald Trump has accused Nigeria of failing to protect Christians from Islamist militants in the northwest.

Nigeria denies discriminating against any religion, saying its security forces target armed groups that attack both Christians and Muslims.

The U.S. carried out strikes targeting Islamic State-linked militants in December, and a small U.S. military team has been operating on the ground to boost Nigeria's intelligence capabilities.

In recent days, several planes carrying U.S. troops and equipment have departed for Nigeria's northern states, according to flight-tracking data reviewed by Reuters.

Major General Samaila Uba, spokesperson for Nigeria's Defence Headquarters, said the troops would train and advise local forces, but not take part in combat.

Earlier this month, Nigeria's military said it expected around 200 more U.S. troops.

Presidential spokesperson Sunday Dare said Nigeria needed "massive support from the U.S. government" in the form of fighter jets and munitions, but declined to provide numbers or a timeframe.

Nigeria's 240 million people are evenly split between Christians, mainly in the south, and Muslims, mainly in the north.

It acknowledges serious security problems, including those from Islamist fighters, but denies that Christians face widespread or systematic persecution.