An oil slick from a rusting cargo vessel that sank in bad weather reached the coast of Sri Lanka's capital on Saturday and threatened a beach resort popular with foreign tourists, officials said.

Coast conservation officials insisted that the spill -- about 10 kilometres (six miles) long -- was "manageable" and could easily be cleaned up, and there were no immediate signs of it affecting wildlife or fish.

However, a thin layer of oil was seen off the coast of Negombo, navy sources said. One of the first tourist resorts that developed in the early 1970s, Negombo is popular with foreign holiday makers.

"We can see a thin layer of oil off the coast, but it has not reached the coast yet," said the navy official in the resort, 40 kilometres (25 miles) north of the capital Colombo.

Earlier in the day, a patch of oil reached the coast of Wellawatte, an area of Colombo popular with local swimmers, the coast conservation department said.

"The spill is manageable and the leak from the sunken ship had stopped from last night," Coast Conservation Department chief Anil Premarathne told AFP. "About 10 or 15 people would be enough for this clean-up."

However, the national Disaster Management Centre (DMC) said it had mobilised 500 volunteers, including soldiers and police, in case of serious damage to the coastline.

The rusting 15,000-tonne Thmothrmopolyseara, a Cyprus-flagged carrier, went down late Thursday after remaining anchored outside a Colombo harbour since 2009 following a dispute over its cargo of steel, local officials said.

DMC director Sarath Kumara said much of the 600 tonnes of oil from the ship had been pumped out before it sank.

The vessel had been detained by Sri Lankan courts following litigation over the cargo of steel, valued at over $300 million, according to local media reports. It was not clear who owned the vessel.

Sri Lanka's merchant shipping authority director Ajith Seneviratne said they had been ready to tow the ship away to a salvage yard in the island's east but were prevented by a court order.