HOUSTON/NEW YORK (Reuters) - A crude oil tanker collided with a service vessel in the Gulf of Mexico near Texas, spilling 18,000 gallons (68,140 liters) of the tanker’s fuel but none of its cargo, the U.S. Coast Guard said on Wednesday.

The accident occurred in rough seas late Tuesday after the tanker finished receiving crude oil from a supertanker too large to enter port 46 miles southeast of Galveston. Ship-to-ship oil transfers, called lightering, are common and often involve a third vessel servicing the operation.

The 820-foot (250-meter) Liberian-flagged, Russian-operated Krymsk, collided with the 166-foot lightering service vessel AET Endeavor, after the Krymsk had finished taking Arabian crude oil from a larger ship, the Vega Star.

One of the Krymsk’s fuel tanks was pierced, spilling No. 6 bunker fuel, officials said. The cargo tanks were undamaged and none of the cargo, 70,475 metric tonnes of crude, spilled, a spokesman for Krymsk operator Novorossiysk said.

The crew transferred the ship’s remaining fuel, about 45,000 gallons, to an undamaged tank, stopping the leak, the spokesman said.

The Endeavor, owned by American Eagle Tankers, was not seriously damaged.

Weather conditions on site - 40-knot winds and 5-foot waves - prevented surface vessels from starting cleanup of the spill, so the Coast Guard used dispersants dropped from airplanes.

The incident is in a zone off Galveston designated for ship-to-ship transfer of crude oil and ship traffic was unaffected by the accident or the spill, the Coast Guard said.

The spill of fuel used to power the Krymsk is relatively small, although No. 6 is heavy and thick compared to gasoline, and posed little environmental threat so far offshore, shipping sources said.

The cause of the accident was under investigation.

The Coast Guard said such accidents are rare in an area where many ships load and unload crude oil for delivery to the largest U.S. refining complex along the Gulf Coast.