The United Nations had already started implementing coronavirus prevention measures at its headquarters this week, including closing to the public, more regular cleaning and cancelling some meetings. (File photo: Reuters)

The United Nations said it will slash its staff presence at its New York headquarters for four weeks, starting Monday, after a Filipino diplomat became the first person at the 193-member world body known to have tested positive for the coronavirus.

The UN announcement on Friday came as host country the United States declared a national emergency over the fast-spreading virus.

UN staff will be available for essential services at the Manhattan campus, such as translation and security, but it was unclear whether the move would prompt meeting cancellations. The 15-member Security Council is due to meet every day next week on issues including Syria, North Korea and Libya.

"Our aim is twofold: reduce our physical presence at United Nations Headquarters; and continue delivering on our mandates," UN Secretary-General Antonio Guterres wrote in a letter to member states on Friday.

"It is essential that we reduce social contact to a minimum and follow the clear instructions of the World Health Organization to minimize the risks of transmitting the infection," he said.

The United Nations had already started implementing coronavirus prevention measures at its headquarters this week, including closing to the public, more regular cleaning and cancelling some meetings.

UN spokesman Stephane Dujarric said the number of people entering daily had fallen by half to about 5,300, between March 4 and March 11 and that there had been a steeper drop on Friday.

Guterres said in his letter that staff not physically required to be in the building would work from home.

"The United Nations remains open for business. But our work will be done from different locations, using different technologies," he wrote.

The UN Children's Fund (UNICEF) also said on Friday that its New York headquarters - across the street from the main UN building - would close for four weeks and staff would work from home after three people showed flu-like symptoms. UNICEF said they had not been confirmed positive for the coronavirus.

A female diplomat from the Philippines mission to the United Nations tested positive for coronavirus on Thursday, according to a note sent to UN missions. Philippines acting UN Ambassador Kira Azucena said the mission was in lockdown and "We are assuming that all of us have been infected."

Several UN meetings planned for Friday were canceled in the wake of the diagnosis.

The World Health Organization has described the coronavirus outbreak as a pandemic. The virus broke out in China's Wuhan city late last year and has so far infected more than 138,000 people from more than 130 countries and territories, killing more than 5,000, according to a Reuters tally.