Christian Solidarity Worldwide (CSW) has expressed concern over the US State Department's decision to remove Nigeria from its Countries of Particular Concern (CPC) list.
Nigeria was placed on the list for the first time in 2020.
Countries are placed on the list if their governments are deemed to have engaged in or tolerated "systematic, ongoing, egregious violations of religious freedom".
CSW warned that religious freedom "remains under serious threat" in Nigeria and that violence has "metastasised", with Christians being routinely kidnapped for ransom, attacked and killed.
"The failure to re-designate Nigeria so soon after it became a CPC is particularly concerning in view of ongoing, egregious religion-related violence by the terrorist group Boko Haram in the northeast, and assailants of Fulani origin in central States, and historic violations targeting Christian communities in the country's Sharia states," it said.
CSW says religious freedom violations have been "insufficiently addressed", and have "increased exponentially" under the current administration.
"Since 2015 thousands have died and tens of thousands have been displaced in a campaign of attacks on predominantly Christian communities in central Nigeria by assailants of Fulani origin for whom religion is either a recruitment factor or a governing ethos," it said.
In Sharia states, underage Christian girls are at risk of abductions and forced conversion and marriage. Other Christians have had their land seized without compensation.
In Kaduna state, the local government demolished 263 buildings in the predominantly Christian Gracelands community in Zaria. The demolished buildings included six churches, a school complex and homes, despite the owners possessing certificates proving their ownership, and at least one court ruling against demolition.
In parts of Nigeria, the violence is not confined to Christians, with Muslim communities from the Hausa ethnic group suffering similar death and displacement.
A local CSW source said the decision to remove Nigeria as a CPC was "very disappointing" and "will give perpetrators the green light to continue violating our rights."
CSW's Founder President Mervyn Thomas said: "CSW is highly alarmed at the US State Department's premature removal of Nigeria from the CPC list at a time when the Nigerian authorities are still failing to protect vulnerable communities, while restricting the ability of journalists, activists and even victims to draw attention to their plight."
He expressed additional concern at the omission of India, Syria and Vietnam from the list, against the advice of the US Commission on International Religious Freedom (USCIRF).
Mr Thomas called on the State Department to "urgently review the situation of freedom of religion or belief in each of these countries, ensuring that economic and other exigences are not prioritised at the expense of the rights, freedoms and lives of individuals and religious communities".
In response to Nigeria's removal from the list, USCIRF Chair Nadine Maenza said: "While the State Department took steps forward on some designations, USCIRF is especially displeased with the removal of Nigeria from its CPC designation, where it was rightfully placed last year."
Crossbench peer Lord Alton has said it is "a decision which makes no sense and should be reversed".