NEW DELHI (Reuters) - Communists, who shore up the central coalition, could pull out of a key government coordination panel if New Delhi compromises on a nuclear deal with the United States, a top left leader said.

Sitaram Yechury, a senior figure in the Communist Party of India (Marxist) (CPI-M), said Prime Minister Manmohan Singh's coalition would come under strong pressure if he was not more "transparent" about the landmark pact.

Although the CPI-M pulling out of the coordination panel -- formed to build a policy consensus between the government and its coalition allies -- would not threaten Singh's government, it could hurt perceptions of its stability.

"All options are open including withdrawing from the coordination committee," said Yechury, whose party has 44 members in the 545-member lower house of parliament and provides the government with a majority from outside the ruling coalition.

Yechury's comments came just ahead of an expected debate in India's parliament over the deal on Thursday, amid concerns that Washington was moving the goalposts in what is seen as a symbol of the new friendship between the once-estranged nations.

It also came ahead of a meeting between Singh and members of the country's nuclear establishment on Wednesday after senior Indian scientists wrote a joint letter to him saying changes to the nuclear pact could hurt India's ability to develop nuclear technology.

The civilian nuclear cooperation deal gives nuclear-armed India access to U.S. atomic fuel and equipment despite New Delhi not having signed the nuclear Non-Proliferation Treaty (NPT).

In return, New Delhi has agreed to international inspections of its civilian nuclear reactors and segregation of its civilian and military programmes.

The deal won the overwhelming approval of the U.S. House of Representatives last month and is due to be considered by the Senate next month. The two houses then have to vote together after negotiations on technical details of the pact.

The deal has come in for strong criticism from U.S. non-proliferation legislators who say Washington has blown a hole in the NPT by making an exception for India.

Indian critics of the deal say Washington was moving the goalposts to surreptitiously curb India's nuclear arms programme through changes to the legislation before Congress.

The prime minister has repeatedly tried to allay these fears by saying he would not compromise on national security and the deal would have to conform to its original parameters.

Some changes proposed by U.S. lawmakers include a clause that would make it mandatory for the U.S. administration to certify every year that India is sticking to the deal's terms.

"We oppose annual certification by the United States which amounts to a virtual good conduct certificate," said Yechury, a lawmaker who is due to speak in parliament on Thursday.

Other amendments proposed by Congressmen include the end of nuclear cooperation if India conducts a nuclear test as well as caps on using spent nuclear fuel.

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