By Stefano Ambrogi

LONDON, March 31 (Reuters) - Curbing greenhouse gas emissions from ships, slashing other air pollutants they generate and cleaning up the world's oceans, top the agenda at a meeting of the world's chief maritime body in London this week.

The U.N. International Maritime Organisation (IMO) meeting, seen as one of the most crucial in years, focuses on how best to reduce harmful ship fuel pollutants like sulphur dioxide emissions and nitrous oxides.

"Shipping should not be allowed to become a scapegoat for those who find it a 'soft target' singling it out from other modes of transport, when data show it as having greener credentials than them," said IMO Secretary-General Efthimous Mitropoulos, opening the meeting on Monday.

The week-long meeting also hopes to speed up policies to tackle growing carbon dioxide emissions emitted by ships, by strict international regulation or through industry-led initiatives.

Other hot topics include:

- Pushing governments to ratify a law stopping the spread of destructive invasive "alien" species through the discharge of ballast water across the world.

The law adopted in 2004 has only been ratified by 12 countries, representing just 3.64 percent of the world's shipping.

In a similar fashion to other IMO conventions, the law will only enter into force 12 months after no less than 30 nations, representing 35 percent of the world's tonnage ratify it.

- Debate a draft text of a new convention that allows for the recycling of merchant ships in a safe and environmentally sound manner.

- Debate the creation of Particularly Sensitive Sea Areas protecting unique and fragile coral reef ecosystems, like one designated for the northwestern Hawaiian Islands.

The crux of the meeting, however, will focus on amending existing marine air pollution laws, that are seen by critics as woefully out of date.

Nations that are signatories to IMO are likely to opt for the use of very low sulphur fuels in special emission-control areas around the world, experts say.

The exact sulphur limit of the ship fuel is still to be agreed, as is a realistic timetable for both the ship industry and the world's oil industry to react to.

The International Chamber of Shipping (ICS) speculates that a portion of the world's 50,000 strong merchant fleet could be required to switch to cleaner distillate fuels in the special areas close to coastlines.

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