A senior State Department official said: “We continuously assess the security environment, and based on our latest review, we determined it prudent to reduce our footprint to essential personnel.
“The embassy remains operational with core staff in place. This is a temporary measure intended to ensure the safety of our personnel while maintaining our ability to operate and assist US citizens.”
About 50 members of US embassy staff have been ordered to leave, with 32 staff members and their families flying out of Beirut airport on Monday, an airport official told Reuters news agency.
A precise reason has not been given for the partial evacuation, but it comes after Trump said “bad things” would happen if Iran did not reach an agreement over its nuclear programme.
The rhetoric has sparked fears of possible retaliation from Tehran, with Iran’s Supreme Leader, Ayatollah Ali Khamenei, posting messages to social media threatening US forces.
Washington holds the Lebanon-based group Hezbollah, which is backed by Iran, responsible for the deadly bombings of a US Marine barracks and the embassy in Beirut in 1983.
On Thursday, Trump said the world would find out “over the next, probably, 10 days” whether a deal would be reached or whether the US would attack Iran.
The US and its European allies suspect that Iran is moving towards the development of a nuclear weapon, something Iran has always denied.
US and Iranian officials said progress had been made after talks between the two sides in Switzerland last week.
Despite this, the US has continued to bolster its huge military deployment near Iran, which includes an array of destroyers, combat ships and fighter jets.
The world’s largest warship, the USS Gerald R Ford, also appears to be heading towards the region.
BBC Verify has previously identified the nuclear-powered aircraft carrier USS Abraham Lincoln in satellite images taken off the coast of Oman, about 700km (430 miles) from Iran.