Millions of Gallons of Oil Spilled Off Northwest Spain : Environment: A Greek tanker crashes on rocks in fog. An explosion starts an inferno near the shore.

A tanker crashed onto rocks outside a fog-shrouded harbor entrance Thursday, breaking apart in heavy seas and spilling millions of gallons of crude oil that threatened the area’s rich fishing grounds.

Hundreds of people fled their homes after an explosion tore the stern loose and set off an inferno only a few yards offshore five hours after the ship ran aground. All 29 crew members were rescued from the Greek ship, officials said.

Curiosity seekers gathered along the rugged coastline of this city of 250,000 to watch huge orange flames boiling from the stern section. Fire licked from oil alongside the tanker, and thick, black smoke billowed high over the city, which is on Spain’s northwestern corner about 280 miles from Madrid.

Authorities said that an oil slick up to a mile wide stretched 12 miles northeastward along the coast of Galicia, as the region is known.

Environmentalists expressed fears for fishing grounds. Fishing is Galicia’s most important industry.

The tanker, the Aegean Sea, was carrying about 23 million gallons of crude from Britain’s North Sea oil-loading terminal at Sullom Voe to a refinery in La Coruna when it ran aground about 5 a.m.

Antonio Gomis, a spokesman for Spain’s Repsol oil company, which chartered the tanker, said two or three of the ship’s nine tanks had ruptured. He said that each tank held an average of 2.7 million gallons of crude oil.

“We believe about two-thirds of the oil is on the ship, and oil from two or three tanks has gone into the sea,” Gomis said.

Officials said the accident occurred close to the spot where the Spanish tanker Urquiola ran aground and exploded in May, 1976, spilling 30 million gallons of oil. Most of that cargo burned up, but oil still caused damage along 130 miles of coastline.

By comparison, the supertanker Exxon Valdez spilled almost 11 million gallons of oil into Alaska’s Prince William Sound in 1989.

The worst oil spill in Europe occurred in March, 1978, off the northwest coast of France when the tanker Amoco Cadiz ran aground and lost 68 million gallons of crude oil.

The environmental group Greenpeace said it is sending three international maritime and oil experts to the site. “An environmental disaster looms,” Greenpeace said in a statement Thursday evening.

Juan Lopez de Uralde, the Spanish spokesman for Greenpeace, said the chance of keeping the spill from fouling the coast is “practically nil” because it is so close to shore.

Fernando Cano, a spokesman for Spain’s merchant marine department, said that floating barriers designed to stop the spill from spreading were being set out but that high winds and rough seas hampered the effort.

Officials expressed hope that high winds might help push the oil slick away from the coast.

A La Coruna city spokesman said heavy smoke had forced the evacuation of 300 residents from a neighborhood near the abandoned Torre de Hercules lighthouse, close to where the vessel ran aground. Officials said the smoke was not toxic.

Cano said the Aegean Sea ran into rocks at the tip of a promontory on the bay as it sailed into port in thick fog and rough seas. He said the vessel’s operating radar should have warned of the proximity of obstacles.