Nigeria has hired a US lobbying firm to help maintain US support and counter what it has called misinformation by Christian Evangelical groups and other US lobbyists about what its government is doing to protect Christians in the country.

US President Donald Trump redesignated Nigeria "a country of particular concern" in November and promised military action if it failed to crack down on the killing of Christians.

The Nigerian government says it is working hard to tackle Islamist and other groups which have attacked both Muslim and Christian civilians and that there is no systematic persecution.

It hired Washington-based consulting firm DCI Group for an initial six months for $4.5-million, with a similar amount due for another six months, a filing with the US Department of Justice dated December 18 and posted on the DOJ website showed.

DCI and the Nigerian presidency did not immediately comment on the filing, which appeared in Nigerian media on Wednesday.

The agreement says DCI would help the Nigerian government "in communicating its actions to protect Nigerian's (sic) Christian communities and maintaining US support in countering West African jihadist groups and other destabilising elements".

Nigeria has faced a long-running Islamist insurgency in the northeast, armed kidnapping gangs in the northwest and clashes between largely Muslim cattle herders and mostly Christian farmers in the middle belt of the country.

The US military carried out an airstrike in northwest Nigeria on Christmas Day, which Trump said had killed multiple Islamic State militants. Trump said in an interview with the New York Times published last week that there could be more strikes if Christians are killed.

On Tuesday, the US military's Africa Command said it had delivered critical military supplies to Nigeria to bolster the West African country's operations, in a sign of increased cooperation between Washington and Abuja.

On its website, DCI says "we are seasoned political operatives, communication strategists" and "experts at re-framing external narratives, and in delivering the right message to the right audience."