By Tommy Wilkes

LONDON (Reuters) -Three more insurance companies including Tokio Marine have left a United Nations-backed net-zero climate alliance, leaving the group with about half the number of members it counted two months ago as insurers take fright at U.S. political pressure.

Some Republican politicians have mounted a campaign against financial institutions collaborating to try to curb carbon emissions, and a group of Republican attorneys general have turned their focus on insurers by accusing them of potentially breaching antitrust laws in the United States.

Japanese insurer Tokio Marine is no longer listed as a member on the Net-Zero Insurance Alliance's (NZIA) website. A spokesperson for Tokio Marine was not immediately available for comment outside of Japanese business hours.

MS&AD Insurance Group, another Japanese firm, said in a statement on Monday it was leaving less than a year after joining. It said it would "continue our journey to achieve Net-Zero by 2050 with our stakeholders".

Spain-based Grupo Catalana Occidente said in a statement it was withdrawing and that it believed it could "continue the path of advancing our sustainability objectives individually, outside the Alliance." It did not elaborate on its reasons for leaving.

The firm added that the NZIA had provided members with help to define their decarbonisation pathway and that it would set "progressive and science-based targets that will enable it to contribute to climate neutrality".

A spokeswoman for the NZIA did not immediately respond to a request for comment.

The NZIA, which was formed in 2019 to get insurers to commit to reducing greenhouse gas emissions in their underwriting portfolios to a net-zero level by 2050, is now down to 17 members, according to its website, against the 28 it had two weeks ago and 30 in late March.

Legal experts say it would be hard to make a case against insurers on antitrust grounds, but cautious international insurers are worried about being sucked into a tussle with U.S. Republicans.

A handful have left since late March but that turned into a mass exodus last week when the NZIA lost at least eight members including Spain's Mapfre, France's AXA - which chaired the alliance - and Japan's SOMPO.

The remaining members of the NZIA, which include Britain's Aviva, Italy's Generali and France's Credit Agricole Assurances, are set to hold more calls this week to decide whether and how the alliance can continue given so many members have quit, sources familiar with the discussions say.

The NZIA is one of several industry climate alliances that exist under the U.N-backed Glasgow Financial Alliance for Net Zero (GFANZ) umbrella group. GFANZ was launched in 2021 ahead of the U.N. climate summit, COP26, in Glasgow.

A spokesperson for GFANZ on Friday said "political attacks" on insurers were damaging insurers' independent efforts to price climate risk.

(Reporting by Tommy Reggiori Wilkes; Editing by Emelia Sithole-Matarise and Mark Potter)