WASHINGTON — Key congressional supporters of closer ties with India have signed a toughly worded bipartisan letter to the Indian prime minister warning of “grave concern” that India’s ties with Iran “have the potential to significantly harm prospects” for a nuclear cooperation deal that President Bush reached with India in 2005.

The letter is noteworthy for its tone and because it was signed by the Democratic as well as Republican leaders of the key congressional panels involved in the issue. It was sent Wednesday, one day after India’s Foreign Secretary Shiv Shankar Menon publicly dismissed reports of closer military cooperation with Iran.

The letter to Prime Minister Singh lists a series of recent meetings between Indian and Iranian officials as indications of growing cooperation between the two countries on military and energy issues. “We must stress that the subject of India’s strengthening relationship with Iran will inevitably be a factor” when the final language on the nuclear agreement must be approved by Congress, the letter said.

Mr. Bush and Secretary of State Rice have pushed hard for the nuclear deal with India, saying that rewriting U.S. laws to allow peaceful nuclear cooperation with India will help usher in a new era in American-Indian relations. But the nuclear accord has been opposed by nonproliferation experts, who fear it will weaken rules preventing the spread of nuclear weapons by allowing the sale of U.S. nuclear technology to a country that has refused to join the nuclear nonproliferation treaty.

When Congress debated a bill to give initial approval to the accord with India, lawmakers considered tying final passage to India’s dealings with Iran, but dropped that provision under pressure from the administration. Congressional aides, speaking on the condition of anonymity in order to not upstage the lawmakers, said anger had been building in Congress over the perception that India stepped up contacts with Iran this year, just weeks after the initial bill was approved by Congress.

Lawmakers “are not just alarmed but actually outraged by India’s outreach to Iran,” one aide said.

The letter was signed by Reps. Tom Lantos, a Democrat of California and chairman of the House Foreign Affairs Committee; Ileana Ros-Lehtinen, a Republican of Florida and the panel’s ranking Republican; Howard Berman, a Democrat of California, Gary Ackerman, a Democrat of New York and chairman of the Middle East and South Asia subcommittee; Mike Pence, a Republican of Indiana and the subcommittee’s ranking Republican; Brad Sherman, a Democrat of California and chairman of the terrorism and nonproliferation subcommittee, and Ed Royce, a Republican of California and that subcommittee’s ranking Republican.

“It is difficult for us to fathom why India, a democracy engaged in its own struggle against terrorism, would want to enhance security cooperation with a repressive government widely regarded as the world’s most active state sponsor of terrorism,” the letter said.

It added that “India’s pursuit of closer relations with Iran appears to be inconsistent with the letter and spirit” of Messrs. Bush and Singh’s announcement of a “global partnership” between the two countries.

Congressional aides said a subtext of the letter is growing concern in Congress that the administration is too eager to wrap up negotiations with India. Mr. Menon negotiated with U.S. officials in Washington this week, and both sides reported that they hope to strike a final deal later this month.