The US has dispatched an aircraft carrier and bomber task force to the Middle East after receiving "clear indications" that Iran was preparing to possibly attack US forces in the region.
National security adviser John Bolton said the move to deploy the USS Abraham Lincoln Carrier Strike Group and a bomber task force to the Gulf was in response to "a number of troubling and escalatory indications and warnings".
He did not provide details, but said the US wanted to send a message that "unrelenting force" would 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 Abraham Lincoln and its strike group of ships and combat aircraft have been taking part in training operations in the Mediterranean Sea.
Secretary of state Mike Pompeo said: "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.
"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", he added: "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."
President Trump's administration has been increasing pressure on the Iranian government after withdrawing from a landmark nuclear deal last year and more recently blacklisting the country's Revolutionary Guard Corps as a terrorist organisation.
Washington has also said it will end exemptions from sanctions for five countries - China, India, Japan, South Korea and Turkey - that are still buying Iranian oil.
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