By Louis Charbonneau and John Irish

UNITED NATIONS (Reuters) - Dozens of nations signed on to a French proposal on Wednesday that the five permanent members of the United Nations Security Council refrain from exercising their veto in cases of mass atrocities and genocide.

Some 75 countries in Europe, Africa, Latin America and Asia backed the proposal, and French officials say they expect more nations to join. There are 193 U.N. member nations.

"We hope there will be more commitments to ensure that these situations, like in Syria, where there are mass atrocities and the U.N. Security Council is paralyzed by a veto, disappear," French Foreign Minister Laurent Fabius told reporters.

He said France, one of the five permanent members with veto power, had already committed to not using its veto in case of mass atrocities and potential genocide, and hoped others would soon follow suit.

None of the other four veto powers - the United States, China, Britain and Russia - have formally signed up for the initiative, according to a map of countries that have joined the initiative provided by France.

"We ... consider that the Security Council should not be prevented by the use of veto from taking action with the aim of preventing or bringing an end to situations involving the commission of mass atrocities," a French-drafted declaration on the veto limitation said.

"We underscore that the veto is not a privilege, but an international responsibility," it added.

In the case of Syria, Russia and China have used their veto four times to block Security Council action, three times to strike down threats of sanctions against the government of Syrian President Bashar al-Assad and another resolution that would have referred the Syria conflict to the International Criminal Court for possible crimes against humanity.

U.N. diplomats say that Britain supports the initiative, while the United States is considering it in a positive light. Both Russia and China, the diplomats say, have suggested they dislike the idea.

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Human rights groups have long called for limits on the use of the veto, saying it enables the five veto powers to shield governments guilty of grave crimes.

"The veto ... should not be used to defend an ally, which is what we are seeing with the Assad regime today," said Kenneth Roth, head of Human Rights Watch, a New York-based advocacy group.

While Russia, with the backing of China, has used its veto power to shield the Syrian government from criticism, the United States has repeatedly blocked council action critical of Israel, Washington's chief ally in the Middle East.

(Reporting by Louis Charbonneau and John Irish; Editing by Jonathan Oatis)