The US State Department said Friday that ongoing fighting between military factions in Sudan made it too risky to try to evacuate embassy personnel from Khartoum.
The Pentagon has mobilised in the east Africa region to bring out US staff from the Sudan capital.
However, "due to the uncertain security situation in Khartoum and the closure of the airport, it's currently not safe to undertake a US government coordinated evac," said State Department spokesman Vedant Patel.
The State Department has sought to gather US staff in one location in the Sudan capital to better protect them from the fighting outside and prepare for an evacuation.
How that would take place is not clear.
On Thursday the Pentagon said it was mustering forces in the region to support an evacuation, presumably to be directed from the US base in Djibouti, more than 700 miles (1,126 kilometres) to the southeast of Khartoum.
"We are deploying additional capabilities nearby in the region for contingency purposes related to securing and potentially facilitating the departure of US Embassy personnel from Sudan," the Pentagon said in a statement.
More than 400 people have been killed after violence erupted Saturday between forces loyal to Sudan's army chief Abdel Fattah al-Burhan and his deputy, Mohamed Hamdan Daglo, who commands the paramilitary Rapid Support Forces (RSF).
A lull in the fighting starting Friday, apparently tied to the Eid al-Fitr holiday, raised the possibility that an evacuation could take place over the weekend.
"We have made very clear to both sides that any attacks, threats or dangers posed to our diplomats are totally unacceptable," Patel said.
Berlin aborted an evacuation attempt for its citizens from Sudan on Wednesday, according to German weekly Der Spiegel.
Three military transport planes, which would have carried around 150 German citizens, headed for the country but were made to turn back, according to the magazine.
South Korea said Friday it was sending a military aircraft and soldiers to standby for evacuating its nationals at the US base for Djibouti.
And Japan said it is preparing to evacuate its nationals from the country. Around 60 Japanese people are in Sudan, including embassy staff, government spokesman Hirokazu Matsuno said.
Japan also has a base in Djibouti for the Japanese Self Defense Forces.