When US President Donald Trump made a surprise threat this weekend on social media to carry out strikes in Nigeria, his defence secretary answered quickly: "Yes sir."

But several Pentagon officials who spoke to Reuters described feeling whiplash from Trump's orders as they try to understand the priorities of an administration that in the past several months has elevated once backburner issues like nuclear testing, democracy in Venezuela and cocaine trafficking.

Pentagon officials broadly expected Trump's administration would prioritise border security, China's growing military might and pressuring NATO allies to do more to stand up to Russia.

But Trump's announcements in the past week on everything from nuclear testing to Nigeria have caught many off guard by appearing to reshuffle Pentagon priorities.

"I think we are all learning about this at the same time," said one US military official, speaking on condition of anonymity, commenting on the Nigeria announcement.

NO SPECIFIC PENTAGON ORDERS, SAYS OFFICIAL

Trump's Saturday night post on Truth Social accused Nigeria's government of allowing the killing of Christians. He ordered the Pentagon to prepare for "possible action," and warned that any strikes would be "fast, vicious, and sweet."

Asked on Sunday if he envisioned troops on the ground or air strikes in Nigeria, Trump told reporters aboard Air Force One: "Could be ... I envisage a lot of things." 

Nigeria is more than 2 000 miles from Djibouti, site of America's only permanent military base in Africa. Experts said the US military needs more resources in the region, and time to build the intelligence needed to attack Islamist groups effectively.

Boko Haram, perhaps the most well-known militant group in Nigeria, was a US priority more than a decade ago when it kidnapped nearly 300 mostly Christian schoolgirls from a northeastern town. Many were raped and forced to convert to Islam. 

But Nigeria faded from focus in recent years, and it was unclear what precisely prompted Trump's Saturday announcement.

"President Trump listens to many of his trusted advisers on any given issue. However, he is the final decision maker on all foreign policy – and here, he was motivated to take action against the existential threat that Christians are facing in Nigeria,” White House spokesperson Anna Kelly said.

Evangelical leader Gary Bauer told Reuters that people increasingly worried about attacks on Christians lobbied Trump before his warning to Nigeria.

Trump's threat came a day after his administration added Nigeria to a "Countries of Particular Concern" list of nations that the US says have violated religious freedoms. 

"I know the president was hearing from a lot of people that he needed to take action," Bauer said.

Some US military members suspected Trump might act after Senator Ted Cruz spoke about Christians in Nigeria in September and October, a defence official told Reuters. Cruz has alleged Nigerian officials allowed what he called a "Christian genocide" involving tens of thousands of killings over the past 15 years.

Still, the defence official was unaware of any specific direction to carry out operations in Nigeria.

Defence Secretary Pete Hegseth is among the vocal Christians inside the Trump administration, even leading prayer sessions for Christians inside the Pentagon on a monthly basis.

SITUATION 'PRETTY DIRE', SAYS ANALYST

With more than 200-million people and around 200 ethnic groups, Nigeria is divided between the largely Muslim North and mostly Christian South.

Insurgents such as Boko Haram and Islamic State West Africa Province have wreaked havoc in the country for more than 15 years, killing thousands of people. But their attacks have been largely confined to the Northeast of the country, which is majority Muslim.

In central Nigeria, mostly Muslim herders and mainly Christian farmers have clashed frequently over access to water and pasture. In the Northwest, gunmen routinely attack villages, kidnapping residents for ransom.

Victoria Coates, a former Trump administration official now at the Heritage Foundation, said as a major oil-producing nation, Nigeria needs to provide security to reassure oil companies they can safely do business there. "The situation is getting pretty dire," she said.

Analysts said most victims of Islamist violence are Muslim. Former US envoy to the region J. Peter Pham, who served during Trump's first term, said Christians suffered disproportionately to their share of Nigeria's population.

"At the very least (Trump) raises awareness of an issue that I think has long been ignored and kind of swept under the rug," Pham said.

TRUMP KEEPS WORLD GUESSING 

In the past week, Trump has thrust into public view issues that had largely been off the radar for most Americans and many national security experts, raising questions about where his foreign policy is headed.

The Nigeria announcement came just three days after Trump shocked the world by announcing on social media he asked the US military to "start testing our Nuclear Weapons", saying the United States could not fall behind Russia and China.

Energy Secretary Chris Wright on Sunday said that would not involve nuclear explosions, and instead would test all the other parts of a nuclear weapon to make sure they were functioning.

Trump's orders prioritising anti-narcotics operations and a military build-up off Venezuela have also kept the world guessing.

Asked if there were plans for a strike on Venezuela, Trump said to reporters on Sunday evening: "How can I answer a question like that?"

"Supposing there were, would I say that to you honestly? 'Yes, we have plans. We have very secret plans,'" Trump said jokingly.