Two of the girls later managed to escape from their abductors. A teacher and a security guard - both Muslim - were also killed in the attack on the secondary school in the north-western Kebbi state.
Earlier this month, Trump said he would send troops into Nigeria "guns a-blazing" if its government "continues to allow the killing of Christians".
Minaj, whose real name is Onika Tanya Maraj-Petty, told an event organised by the US embassy to the UN in New York that calling for the protection of Christians in Nigeria was "not about taking sides or dividing people... but about uniting humanity".
"This is about standing up in the face of injustice. It's about what I've always stood for," she added.
The 42-year-old Trinidad-born rapper, who grew up in the US, has previously spoken of her Christian faith. She thanked Trump for "prioritising this issue and for his leadership".
The Nigerian government has pushed back on these claims, describing them as "a gross misrepresentation of reality".
An official said that "terrorists attack all who reject their murderous ideology - Muslims, Christians and those of no faith alike".
Other groups monitoring political violence in Nigeria say most victims of the jihadist groups are Muslims.
The country's 220 million people are roughly evenly split between followers of the two religions, with Muslims in the majority in the north, where most attacks take place.
On Wednesday, Nigeria police in the south-western Kwara state confirmed a deadly attack on a church in the town of Eruku, where gunmen opened fire on worshippers the previous day, killing two people and abducting several others.
Local media say armed men, identified by residents as bandits, stormed the Christ Apostolic Church during an evening programme on Tuesday evening, shooting the pastor and rounding up the faithful at gunpoint.
Images and short video clips - believed to be from the church's CCTV cameras - have circulated widely online, showing terrified people scrambling for safety, including an elderly woman seen desperately trying to escape the gunmen.
