Live news video showed a heavy police presence outside the fence around the compound on Manhattan’s East Side as the man paced back and forth.
Police were trying to establish a dialogue with the man, who appeared to be his 60s, a New York Police Department spokesman said.
Traffic was being diverted from the area, the NYPD tweeted.
“There is currently NO THREAT to the public,” it said.
“We have absolutely zero indication that this person is a staff member or former staff member or in any way linked to the U.N.,” U.N. spokesperson Stephane Dujarric told reporters.
Dujarric said an initial full lockdown was partially eased by early afternoon with the reopening of a separate entrance.