A Japanese disaster relief team dispatched in response to the leakage of fuel oil from a Japanese-owned freighter that ran aground off Mauritius said Friday it cannot estimate when the cleanup operation could be completed.

"The spilled oil has stuck to coastal mangroves where it is hard to remove," a member of the team said in an online press briefing hosted by the Japan International Cooperation Agency via the videoconferencing app Zoom.

Using chemicals to dissolve the oil is not a viable option as it may harm the ecosystem, the team said, adding that it was not presently aware of any damage to underwater coral.

Supplied photo of a Japanese-owned bulk carrier stranded on a reef off the coast of Mauritius taken from Mauritius Prime Minister Pravind Jugnauth's official Twitter account. (Kyodo)

The team, which consists of four experts from the Japan Coast Guard and one official each from the Foreign Ministry and JICA, arrived in the Indian Ocean island nation on Monday.

A recent survey of the area found that oil had spread to around 10 kilometers north of where the ship had ran aground near Pointe d'Esny, an area designated as a wetland of international importance under the Ramsar Convention.

More than 1,000 tons of fuel oil has leaked from the vessel, according to its operator Mitsui O.S.K. Lines Ltd.

The team expects to finish removing all the oil remaining inside the vessel within the next few days, weather permitting.

The Panama-flagged bulk carrier Wakashio, owned by Nagashiki Shipping Co., was carrying a total of some 3,800 tons of fuel oil when it ran aground on July 25. The oil began to leak last week when one of the five fuel tanks cracked.