LONDON - The United Nations' shipping agency is under pressure this week to agree on a plan to cut carbon emissions from the sector, following years of slow progress, but the strategy could fall well short of what is required to limit global warming.

The shipping sector, along with aviation, avoided specific emissions cut targets in a global climate pact agreed at the end of 2015, which aims to limit a global average rise in temperature to "well below" 2 degrees Celsius from 2020.

Shipping accounts for 2.2 percent of world CO2 emissions, according to the International Maritime Organization (IMO), the U.N. agency responsible for regulating pollution from ships.

This is around the amount emitted by Germany, according to the latest EU data available, and is predicted to grow significantly if left unchecked.

The European Commission estimates that air and marine transportation could contribute as much as a third of all emissions by 2050.

The IMO is meeting in London this week to develop a initial strategy to cut emissions. A final plan is not expected until 2023.

The strategy should include a global emissions reduction goal for the sector which is in line with temperature limits agreed under the global climate deal and a list of short-, mid- and long-term measures to ensure this goal is met.

A spokesman for the European Commission said EU countries, along with the Marshall Islands, support a goal of cutting emissions by 70 to 100 percent by 2050, compared with 2008 levels.

Norway has proposed a 50 percent cut by 2050, while another proposal backed by Japan - which is also chairing this week's talks - and other countries aims for a 50 percent cut by 2060.

These targets would not be ambitious enough to keep global temperature rise to "well below" 2 degrees C, some environmental groups and countries argue.

Countries such as Brazil, Saudi Arabia and Panama are also opposed to the EU's proposal of a loftier target, sources say.

A report by the OECD's transport think tank International Transport Forum last week said the maritime sector could be de-carbonised fully by as early as 2035.

The 70-100 percent reduction proposal from some European countries is unlikely to achieve broad support, said the chairman of the International Chamber of Shipping (ICS), which represents more than 80 percent of the world's merchant fleet.

"While ICS does not fully agree with them in every respect, alternative proposals made by China and Japan merit serious consideration and could form the basis of a possible compromise," said ICS chairman Esben Poulsson.

The Marshall Islands, the second largest flag registry in the world, is one of the countries most vulnerable to climate change. Its environment minister David Paul said on Monday there was "simply no credible reason to hesitate any longer".

The IMO has adopted mandatory rules for new vessels to boost fuel efficiency as a means of cutting CO2 from ship engines.

There have been discussions for years on wider CO2 cuts but progress has been slow, prompting calls for potential unilateral measures to be taken at regional or national level.

IMO Secretary-General Kitack Lim said last week "postponing the adoption of the initial strategy, to a future session..., should not be an option".

IMO members have set themselves a deadline to adopt the initial strategy at this week's meeting, but they could still postpone it to a later meeting if no decision is reached.

"This week is really crucial for the credibility of IMO," said member of the European Parliament Bas Eickhout.

"If this week they fail again, countries will have to take their own actions," he added.

(editing by David Evans) ((nina.chestney@thomsonreuters.com; +44 (0)207 542 8071; Reuters Messaging: nina.chestney.thomsonreuters.com@reuters.net))