(Updates with detail, comment)
LONDON, April 13 (Reuters) - The United Nations shipping agency reached an agreement on Friday to cut carbon emissions following years of slow progress, delegates said.
The compromise plan, which will cut emissions by at least 50 percent by 2050 compared with 2008 levels, fell short of more ambitious targets, they added.
Delegates said opposition from some countries - including the United States, Saudi Arabia and Panama - had limited what could be achieved at a session of the International Maritime Organization (IMO) this week in London.
"The IMO should and could have gone a lot further," said Bill Hemmings, shipping director with green campaigners Transport & Environment.
"This decision puts shipping on a promising track."
Greenpeace International political adviser Veronica Frank said the plan was "far from perfect but the direction is now clear - a phase-out of carbon emissions."
"This decarbonisation must start now and targets improved along the way, because without concrete, urgent measures to cut emissions from shipping now, the Paris ambition to limit warming to 1.5 degrees will become swiftly out of reach, Frank said.
Shipping association BIMCO in contrast described it as a "landmark agreement."
The shipping sector, along with aviation, avoided specific emissions-cutting targets in a global climate pact agreed in Paris at the end of 2015, which aims to limit a global average rise in temperature to "well below" 2 degrees Celsius from 2020.
European Union countries along with the Marshall Islands, one of the world's biggest ship registries, had supported a goal of cutting emissions by 70 to 100 percent by 2050, compared with 2008 levels.
British-based research group InfluenceMap said an emissions cut of 70 percent would have been much closer to what is needed if shipping is to be in-line with the goals of the Paris agreement.
Shipping accounts for 2.2 percent of world CO2 emissions, according to the 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.
A final IMO plan is not expected until 2023. (Editing by Dale Hudson)