The US embassy has been calling on US citizens for several days to take commercial flights to leave the country, where rebel groups no longer rule out marching in the capital Addis Ababa.
The embassy offers loans to those who are unable to purchase a ticket immediately.
“We are doing that not because we are pessimistic about the prospects for peace, but because we are practical,” said State Department spokesman Ned Price.
He said he feared a “misperception” among the public “that what we saw in Afghanistan is something that the US government can undertake anywhere and everywhere in the world”.
In mid-August, when the Taliban captured Kabul before the US military had completed its withdrawal from Afghanistan, Washington sent thousands of soldiers back to the capital's airport to improvise an unprecedented evacuation operation.
In just over two weeks, with the help of its allies, the US military set up an airlift and evacuated more than 123,000 people — Americans and other foreigners, but also thousands of Afghans who feared reprisals from the country's new militant masters, especially those who had worked with westerners in the past.
'Afghanistan was unique'
However, the government of President Joe Biden was criticised for failing to plan ahead more for the operation, and for leaving some Americans behind after the withdrawal of foreign forces.
The State Department announced an internal investigation to determine how it could have better anticipated and organised such a huge evacuation.
“What we did in Afghanistan was unique,” Mr Price said on Monday, noting that “a military airlift of nearly 125,000 people … is not something the US government can replicate elsewhere”.
“There is no reason for Americans to wait until the last minute” to leave the Horn of Africa country on commercial flights, he said.