The United Nations said Thursday it has sent a team of international experts to Bangladesh to help clean up the world's largest mangrove forest, more than a week after it was hit by a huge oil spill.

Thousands of litres of oil have spilt into the protected Sundarbans mangrove area, home to rare Irrawaddy and Ganges dolphins, after a tanker collided with another vessel last Tuesday.

A team from the United Nations Disaster Assessment and Coordination (UNDAC) has arrived in the capital Dhaka to support Bangladesh's "cleanup efforts of the oil spill in the Sundarbans", a statement from the UN said.

Experts have slammed authorities for failing to organise a proper clean-up effort of the oil spill, which has now spread 350 square kilometres (135 square miles) inside the delicate mangrove forest area.

Until now, the forest department was relying on villagers and fishermen to scoop up the thick tar from the water and river banks with sponges and pans.

The UN team will help in the "ground work in coordination with the government" and "will also conduct an assessment and advise on recovery and risk reduction measures", it said, adding the team had been sent in response to a request from Bangladesh.

The European Union and United States, Britain and France are supporting the UN effort, it added.

The UN expressed concern over the disaster, urging Dhaka to impose a "complete ban" on the movement of commercial vessels through the 10,000 square kilometres ( 3,850 square miles) forest that straddles the border between Bangladesh and India and is home to a number of rare animals including the endangered Bengal tigers and Irrawaddy dolphins.