Washington DC - Secretary of State Marco Rubio said Wednesday the US would restrict visas over persecution of Christians in Nigeria, where President Donald Trump has threatened military action.

The US already restricts visas for human rights abuses, and Trump has sharply curtailed visas in general, especially for people from developing countries.

Rubio, in a statement, said the US would restrict issuing visas to people who "have directed, authorized, significantly supported, participated in or carried out violations of religious freedom."

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Rubio called the step "decisive action in response to the mass killings and violence against Christians by radical Islamic terrorists, Fulani ethnic militias, and other violent actors in Nigeria and beyond."

In November, Trump stunned many by posting on social media that the US was ready to take military action in Nigeria to counter the killing of Christians.

US officials, while not contradicting Trump, have since instead emphasized other US actions on Nigeria, including security cooperation with the government and the prospect of targeted sanctions.

A senior Nigerian delegation recently visited Washington and announced a readiness to "deepen security cooperation" with the US.

Religiously diverse Nigeria is the scene of a number of long-brewing conflicts that have killed both Christians and Muslims, often indiscriminately.

Evangelical Christians, a key constituency for Trump, have led charges for years of a targeted campaign of persecution against Christians in Nigeria.

Many scholars say the reality is more nuanced, with conflicts rooted in struggles for scarce resources rather than directly related to religion.