A major shipping channel in New York harbor was closed to vessel traffic Thursday after an 811-foot oil tanker ran aground in the early morning, spilling 260,000 gallons of heavy, black oil into the water, the U.S. Coast Guard said.

The waterway, the Kill Van Kull, separates New Jersey from Staten Island and is the principal access route for containerships entering Newark Bay and the immense container terminals of Newark and Elizabeth, N.J. No serious disruption of vessel traffic was expected, however, since an alternate access route, the Arthur Kill, remained open to shipping traffic.The spill, which the Coast Guard classed as a major oil spill, was the fifth significant spill to occur in the narrow, winding kills this year.

The tanker BT Nautilus ran aground at about 5:15 a.m. as it passed a nearby refinery and began leaking No. 6 oil into the waterway, said Coast Guard Petty Officer Jim McGranachan.

Officials were unclear about what caused the oil to leak.

''It ran aground. The No. 4 was damaged. Until they are able to remove the oil from that tank," it will keep spilling into the water, said Gene Hoff, chief public affairs specialist for the Coast Guard.

Mr. Hoff said a pilot had been on board the Nautilus at the time of the grounding.

Officials didn't know how large a slick was created by the leaking fuel. About 350,000 barrels of oil were on board the tanker, owned by Nautilus Motor Tanker Co., London.

Ian Michaels, a spokesman for the New York City Department of Environmental Protection, called the spill area "environmentally sensitive

because it is ringed with all these oil refineries. It's extremely sensitive

because of all the abuse it's taken in the last few months."

Absorbent booms were set out to contain the spill and other cleanup crews were called in to assist with the cleanup, Petty Officer McGranachan said.

The owner of the tanker said in a statement: "At the time of grounding BT Nautilus was approaching the berth under guidance of a licensed pilot and with the assistance of tugs attached at both the forward and after ends. No mechanical malfunctions were reported on the vessel, which was within the draft limitation of both the approach channel and berth."

''All appropriate authorities were notified and containment measures were immediately initiated," the statement said. An industry source said a docking pilot of Greenwich, Conn.-based Moran Towing Corp. was on the ship at the time of grounding.

The Kill Van Kull and the Arthur Kill, a second shipping channel between New Jersey and Staten Island, N.Y., have been plagued by several major oil spills this year.

On Jan. 1-2, 567,000 gallons of oil leaked into the Arthur Kill from a ruptured Exxon underwater pipeline.

On Feb. 28, 30,000 gallons of heating oil spilled into the Kill Van Kull

from a barge anchored in Bayonne. A day later, nearly 4,000 gallons of heavy crude oil spilled into the Arthur Kill at Exxon's refinery in Linden.

About a week later, more than 100,000 gallons of heating oil was dumped in the Arthur Kill near Linden from a barge that exploded and caught fire.