LONDON/ATHENS (Reuters) -A group of top shipping companies including leading Greek players said on Thursday they want changes to a United Nations deal tabled for adoption in October that seeks to cut marine fuel emissions, adding complications to the draft accord after U.S. opposition.

Global shipping accounts for nearly 3% of the world's carbon emissions, and the proposed deal is crucial to speed up decarbonisation through a bigger regulatory framework.

The group - including some of the world's biggest oil tanker companies such as Cyprus-based Frontline and Saudi Arabia's Bahri - said they had "grave concerns" about the so-called Net-Zero Framework proposed for adoption next month at the U.N.'s International Maritime Organization environmental committee.

"As it stands, we do not believe the IMO NZF will serve effectively in support of decarbonising the maritime industry ... nor ensure a level-playing field as intended," the companies told Reuters in a joint statement on Thursday.

"We believe that critical amendments to the IMO NZF are needed, including the consideration of realistic trajectories ... before adoption can be considered."

In April, countries struck a draft agreement that would impose a fee on ships that breach global carbon emissions standards.

The United States has told countries to reject the deal or face tariffs, visa restrictions and port levies, sources told Reuters in September.

The joint statement said it was essential that any accord avoided "excessive financial burdens and inflationary pressure to the end-consumer".   

IMO Secretary-General Arsenio Dominguez said he was confident the deal would be adopted next month.

"I base that on the track record of the organization, on the co-operation that we all have, the understanding that we still have some challenges and some concerns particularly to address," he told a Capital Link shipping conference in London on Tuesday. 

Greek Shipping Minister Vassilis Kikilias told Dominguez during London International Shipping Week earlier this week that improvements were required.   

"The minister underlined that he shares the shipping industry's concerns," the shipping ministry said in a statement.

Sources have told Reuters that it was unclear whether the deal could go through if opposition increased or if there were abstentions by IMO member countries. 

About 90% of the world's trade is conducted by sea, and emissions are set to soar without an agreed mechanism.

The statement was also co-signed by Capital Group, TMS Group, Centrofin, Marine Trust, Trust Bulkers, Common Progress, Dynacom, Dynagas, Emarat Maritime, Gaslog, Hanwha Shipping, Angelicoussis Group, Seapeak and Stolt-Nielsen.