The United States has withdrawn from talks in London looking at advancing decarbonization in the shipping sector, and Washington will consider "reciprocal measures" to offset any fees charged to U.S. ships, a diplomatic note said.

Delegates are at the UN shipping agency's headquarters this week for negotiations over decarbonization measures aimed at enabling the global shipping industry to reach net zero by "around 2050."

An initial proposal by a bloc of countries including the European Union, which was submitted to the UN's International Maritime Organization (IMO), had sought to reach agreement for the world's first carbon levy for shipping on greenhouse gas (GHG) emissions.

"The U.S. rejects any and all efforts to impose economic measures against its ships based on GHG emissions or fuel choice," according to a diplomatic demarche sent to ambassadors by the U.S.

"For these reasons the U.S. is not engaging in negotiations at the IMO 83rd Marine Environment Protection Committee from 7-11 April and urges your government to reconsider its support for the GHG emissions measures under consideration."

It was not clear how many of the IMO's 176-member countries received the note.

"Should such a blatantly unfair measure go forward, our government will consider reciprocal measures so as to offset any fees charged to U.S. ships and compensate the American people for any other economic harm from any adopted GHG emissions measures," the note from Washington said.

Washington also opposed "any proposed measure that would fund any unrelated environmental or other projects outside the shipping sector," the note added.

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U.S. officials in Washington did not immediately comment when contacted late on Tuesday.

The IMO had not yet received any communication, an IMO spokesperson said on Wednesday.

Shipping, which transports around 90 per cent of world trade and accounts for nearly three per cent of the world's carbon dioxide emissions, has faced calls from environmentalists and investors to deliver more concrete action, including a carbon levy.

IMO discussions continued on Wednesday despite the move by the US, delegates involved said.

"The U.S. is one of 176 IMO member states. While their impact on this process is considerable, we cannot afford to let a squall from one nation blow all of us off course," said Albon Ishoda, the Marshall Islands special envoy for maritime decarbonization.

"In this time of instability in the markets, a clear direction from this IMO meeting is needed more than ever. We are going to continue negotiating with those that are prepared to talk," said Ishoda.

China and Brazil are among a number of countries that opposed a flat carbon levy on shipping as they argued it would  penalize trade reliant emerging economies.

In January, U.S. President Donald Trump ordered the United States to withdraw from the Paris climate agreement for a second time, placing the world's top historic emitter of greenhouse gases outside the global pact aimed at pushing nations to tackle climate change.