Italian authorities said Saturday they were monitoring an oil slick off the country's picturesque Riviera coast, but said the risk of a new spill into the Mediterranean was limited.

The slick, which was two kilometres (1.25 mile) long and 500 metres wide, was moving slowly westwards from waters off Genoa, raising fears it could pollute holiday beaches just as the tourist season begins.

The oil is believed to have come from a pipeline leak last Sunday at a refinery at Bussala, an outlying suburb of the northwestern Italian city, that spilled large quantities of crude into the Polcevera river.

The refinery's owner, Iplom, insisted that the leak was contained, but one of the barriers erected on the river gave way on Saturday morning after heavy overnight rain, pushing crude into the sea.

After declaring a local state of emergency, Genoa's port authority and the government said that back-up floating barriers in the mouth of the river had done their job.

"The situation is delicate but under control," said Graziano Delrio, the minister for transport and infrastructure.

Genova is located in the middle of the Italian stretch of the Riviera, close to the famous resort of Portofino and several protected areas of outstanding natural beauty, including the Cinque Terre region.

The maritime environment is also highly prized with the coastal waters providing valuable breeding grounds for sea-life as well as supporting a fishing fleet which serves the local restaurant trade.

Genoa mayor Marco Doria said the large slick and several smaller ones spotted by fishermen and coastguards had presumably been caused by the refinery leak on April 17.

"From an environmental point of view I am calm," said Gianfranco Benedetti, Iplom's local safety officer.

"There was no new leak into the sea. There is not much stuff left in the Polcevera, most of it has been extracted by gully suckers," he told the AGI newswire.

Maritime authorities in Toulon, southwest France, said they had immediately ordered heightened monitoring in the area, with the navy dispatching a Falcon 50 plane to assess the situation.

"The flight detected no slick off France and in international waters up to Savona," just east of Genoa, they said in a statement.

Further French surveillance flights are scheduled for Sunday and the coming days.