Hundreds of students and fishermen were working Friday to clean up an oil spill on India's southern coast that campaigners say threatens the turtles that nest there every year.

The Indian Coast Guard said around 35 kilometres (21 miles) of coastline off the southern city of Chennai had been affected by the spill which occurred when two ships carrying fuel collided last week.

Campaigners and fishermen have accused the government of being slow to contain the damage from the spill, the scale of which has only emerged in recent days.

"What ought to have been a localised incident has now become a regional one because of the culture of denial," said Nityanand Jayaraman, a Chennai-based environmental activist.

Jayaraman told AFP it was the peak nesting season for Olive Ridley turtles, which swim to the beaches of South India to lay eggs after mating at sea.

"The key thing is not technology but honesty. You need to reveal the true extent of the damage otherwise it leads to a false sense of complacency. With oil spills long-term effects are certain," he said.

Olive Ridleys are most abundant of all sea turtles around the world, according to WWF India, but their numbers have been declining and the species is recognised as vulnerable by the IUCN Red list.

Their unique mass nesting, where thousands of females come together on the same beach to lay eggs, is a major tourist attraction.

It is not known how many turtles have been affected, but AFP pictures showed workers in safety suits removing a dead turtle from the shore this week.

Local fishermen also criticised the speed of the official response to the spill.

"They (authorities) didn't remove it immediately, and so now it has spread across all beaches, including Marina (beach)," K. Bharati of the Fishermen's Welfare Association in South India said, referring to a popular local beach.

Volunteers wearing gloves could be seen scooping up the thick tar with shovels off Kamarajar Port in Chennai, the capital of Tamil Nadu state.

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"We have tried all kind of technology and found that only manual cleaning is possible. So we have deployed more than 500 people, they are working really hard," M.A. Bhaskaran, chairman of Kamrajar Port, told reporters.

Oil spills are not uncommon in peninsular India, which saw one of the worst leakages in 2013 when a gas pipeline off the western Mumbai coast spewed at least 1,000 litres of crude oil into the sea.

In 2010, two merchant ships collided off the Mumbai coast again, spilling over 800 tonnes of oil and damaging mangroves along the coastline.