The decision takes place amid a tense ceasefire announced Monday following days of fierce fighting between President Salva Kiir's troops and rebels backing former Vice President Riek Machar.
Meanwhile, Washington said it was organizing flights to evacuate non-essential staff and U.S. citizens wishing to leave South Sudan, since commercial flights remain cancelled though charter flights are evacuating hundreds of aid workers and other foreign citizens.
Renewed fighting between the government and rebels has led to a surge in violence in the world's youngest country, leaving at least 270 people dead since last Friday and risking the country's plunge into a new civil war.
Both the government and rebels have been accused of perpetrating ethnic massacres, recruiting and killing children and carrying out widespread rape, torture and forced displacements of populations to purge their opponents from areas.
Thousands of people have been killed and more than two million forced from their homes because of clashes between the rival factions, according to international agencies that have also warned about the population facing a dire threat from increasingly severe food insecurity.
South Sudan gained its independence from Sudan in 2011 following a referendum passed with 98.8 percent. The vote followed a peace agreement between both countries that ended more than 20 years of civil war.