The center said it was expected to remain an "extremely dangerous" category 4 or 5 hurricane when it passes near or over the Virgin Islands overnight, and Puerto Rico on Wednesday.
A hurricane warning is in effect for the U.S. Virgin Islands, the British Virgin Islands, Puerto Rico, Culebra, Vieques, the Dominican Republic from Cabo Engano to Puerto Plata, the Turks and Caicos Islands and the southeastern Bahamas. Puerto Rico has not experienced a storm of this magnitude since 1932, and the National Hurricane Center says Maria will be "an extremely risky and life-threatening situation" for the island territory.
Reports from reconnaissance aircraft indicate that the area of hurricane-force winds has increased in size.
Some people fleeing the island landed at Kennedy Airport on Tuesday night.
Greg Olsen suffered broken foot in Week 2
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San Juan prepares for Hurricane Maria.
"We will need help, my friend, we will need help of all kinds", he said.
The impact on the US territory is expected to be devastating, forecasts said.
The islands of Guadeloupe and Dominica were hammered by catastrophic winds and rain for hours Monday.
The US territory, which has a population of about 3.4 million people, avoided a direct hit from Irma, but the storm knocked out power for 70% f the island, and killed at least three people.
"They have got generators, they bought a lot of water a lot of canned foods, Pampers", East Harlem resident Jose Morales said.
"We have not experienced an event of this magnitude in our modern history", Ricardo Rossello, governor of Puerto Rico, said in a televised message on Tuesday.