A huge oil tanker burning off Italy was rocked by a series of violent explosions Saturday, increasing fears of a major environmental disaster in the Mediterranean Sea and prompting the government to declare a national emergency.

The new blasts aboard the 109,000-ton Cypriot tanker Haven off northwestern Italy's riviera threatened to loose more than 120,000 tons, or about 1 million barrels, of the Iranian crude oil estimated to remain in its tanks.And 90 miles to the north, a blast struck the Agip Abruzzo, carrying an estimated 75,000 tons of oil. The ship was hit by a ferry Wednesday off the northwestern coast of Italy in a crash that killed 138 people and sparked a massive fire.

There were no injuries in the blasts Saturday. The cause of the explosions were under investigation.

Italian Environment Minister Giorgio Ruffolo told reporters Friday that if the Haven's cargo spilled, the result would be "the most serious ecological disaster that ever happened in the Mediterranean."

The fire aboard the Haven began around noon Thursday when a terrific explosion blasted a hole amidships and almost cut the supertanker in two. Five Haven crewmen, including the tanker's Greek skipper, were killed and most of the 31 crew members who were rescued suffered serious burns.

A communique issued after the first meeting of Prime Minister Giu-lio Andreotti's new government in Rome Saturday said Andreotti delivered a special decree declaring the state of national emergency, believed Italy's first since the end of World War II.

Political sources said the national emergency would permit full mobilization of pertinent resources to cope with the potential disaster.

An international campaign to help Italy began with the arrival in Genoa of French Environment Minister Brice Lalonde and Jacques Mellik, the French minister responsible for matters concerning the sea.

They were to coordinate with Italian officials an attempt by teams of Italian and French technicians to contain the spillage and clean up the pollution. The French ministers feared the crude could reach French Mediterranean resorts adjoining the Italian riviera.