The United States' latest sabre-rattling towards Iran was reportedly triggered by photographs of missiles being loaded onto small boats by paramilitary forces.
The missiles "presented a different kind of threat than previously seen from Iran" the New York Times reported, citing three anonymous Trump administration officials.
The US has sent an aircraft carrier and B-52 bombers to the Persian Gulf where the Iranian boats were spotted.
National Security Adviser John Bolton, a hawk who has advocated for regime change in Iran and a number of other countries whose administrations are at odds with the US, believes Iran is planning a strike on US forces in the area. This has been disputed by US allies.
"There's been no increased threat from Iranian backed forces in Iraq and Syria," British Maj Gen Chris Ghika said on Wednesday (local time).
"If you take a look at what other hot spots and sources of conflict are there in this region, then we certainly do not need one thing at the moment: an additional fuse," said German Foreign Minister Heiko Maas.
Tensions are high following drone attacks on Saudi oil facilities earlier this week, carried out by Iran-backed Houthi rebels in Yemen. It's not clear if Iran had anything to do with the strikes. Saudi Arabia is a key US ally in the region.
US President Donald Trump has denied wanting armed conflict.
"Hope not," he said on Friday (NZ time) when asked if the US was going to war with the Islamic republic.
The Washington Post reported Trump is concerned his advisers are pushing him towards warfare.
"He's the one the American people elected," White House spokeswoman Sarah Sanders told Fox News.
"He's going to take in the information and the guidance from all of his national security team and he then will make a decision on what he thinks is the best and safest thing for the American people."
US House Speaker Nancy Pelosi, a Democrat who frequently clashes with the President, welcomed Trump's reluctance.
"I like what I hear from the President - that he has no appetite on this," she said, adding that Trump had no Congressional authorisation to declare war anyway.
Iran and the US haven't had formal diplomatic relations in nearly four decades, after the 1979 revolution.