By Ahmed Kingimi

ABUJA, Dec 8 (Reuters) - Nigeria's government has rescued 100 schoolchildren who were abducted last month from St. Mary's Catholic School in central Nigeria in one of the country's biggest mass kidnappings to date, a Christian group said on Monday.

Nigerian authorities have yet to comment publicly on the rescue, and it was unclear whether the children were freed through negotiations with their captors - or payments to them - or in a security raid.

There have been a wave of school abductions since 2014, when Boko Haram militants kidnapped 276 girls from Chibok in the northeastern Borno state. Ransoms have often been arranged by the families of those kidnapped in previous cases, although officials in Nigeria rarely discuss such payments.

The Christian Association of Nigeria (CAN) spokesperson in Niger state, Daniel Atori, told Reuters the St. Mary's Catholic School rescue had been confirmed by government officials earlier on Monday.

More than 300 children and 12 school staff had been kidnapped on November 21 by gunmen at the Catholic boarding school in Papiri, a remote hamlet in Niger state. Fifty pupils escaped in the following hours, but until Monday there had been no update on the whereabouts or condition of the other children.

Some of those abducted were as young as 6, and the incident sparked nationwide outrage over worsening insecurity in northern Nigeria, where armed gangs frequently target schools for mass kidnappings to extract ransom payments.

The St. Mary's attack threw a spotlight on Nigeria at a time when the country has been under pressure from U.S. President Donald Trump over its alleged ill-treatment of Christians.

A U.S. congressional delegation on Sunday met with Nigeria's national security adviser Nuhu Ribadu in Abuja, where talks "focused on counter-terrorism cooperation, regional stability and strengthening (the) Nigeria-U.S. security partnership," Ribadu said.

(Reporting by Ahmed Kingimi in Maiduguri; Writing by Elisha Bala-Gbogbo; Editing by Toby Chopra and Hugh Lawson)