A White House decision to send an aircraft carrier to the Middle East is meant to serve as a warning to Iran.
National security adviser John Bolton said USS Abraham Lincoln’s move to the region is a response to "a number of troubling and escalatory indications and warnings", although he didn’t give any details.
A defence official told the Associated Press there were "clear indications" that Iranian and Iranian proxy forces were preparing to possibly attack US forces in the region.
He claimed the Pentagon approved the deployments and that US forces at sea and on land were thought to be the potential targets.
Why is this decision significant?
Bolton said on Sunday night that the US is deploying the USS Abraham Lincoln Carrier Strike Group and a bomber task force to the US Central Command region, an area that includes the Middle East.
The Abraham Lincoln and its strike group of ships and combat aircraft have been operating in the Mediterranean Sea as recently as last week.
Bolton's reference to the Central Command area would mean the Lincoln is headed east to the Red Sea and perhaps then to the Arabian Sea or the Persian Gulf.
The US Navy currently has no aircraft carrier in the Persian Gulf.
That move was made to “deny the regime its principal source of revenue”, having already designated Iran's Revolutionary Guard a terrorist group, the first ever for an entire division of another government.
Trump withdrew from the Obama administration's landmark nuclear deal with Iran in May 2018 and then reimposed punishing sanctions targeting Iran's oil, shipping and banking sectors.
Secretary of State Mike Pompeo and Bolton have in recent months spoken stridently about Iran and its "malign activities" in the region.
Bolton said the US wants to send a message that "unrelenting force" will meet any attack on US interests or those of its allies.
"The United States is not seeking war with the Iranian regime, but we are fully prepared to respond to any attack, whether by proxy, the Islamic Revolutionary Guard Corps, or regular Iranian forces," he said.
The Pentagon had no immediate comment on the Bolton statement.
Speaking to reporters while flying to Europe, Pompeo said the actions undertaken by the US had been in the works for a little while.
"It is absolutely the case that we have seen escalatory actions from the Iranians and it is equally the case that we will hold the Iranians accountable for attacks on American interests," Pompeo said.
"If these actions take place, if they do by some third-party proxy, a militia group, Hezbollah, we will hold the Iranian leadership directly accountable for that."
Asked about "escalatory actions", Pompeo replied: "I don't want to talk about what underlays it, but make no mistake, we have good reason to want to communicate clearly about how the Iranians should understand how we will respond to actions they may take."