As India negotiates a civil nuclear deal with the US, its ties with Iran are coming under closer scrutiny with Washington saying that New Delhi should use its influence to ask Tehran to "behave responsibly" and not go ahead with its alleged nuclear weapons programme.
"In terms of what we say and while what we have discussed with our Indian counterparts, I know that we have raised Congressional concern about their cooperation with Iran," State Department's Deputy Spokesperson Tom Casey told reporters in reply to a question on Thursday.
"We continue to encourage the Indians to use what influence they have with the Iranians to press them to comply with UN Security Council Resolutions and to behave responsibly in a wide variety of areas," he added.
The state department official, however, said he had not seen a letter written by some legislators led by Tom Lantos, chairperson of the House Foreign Affairs Committee, to Prime Minister Manmohan Singh on New Delhi's ties with Tehran.
"We have not seen this letter and I'm not sure whether the Indians have had a chance to look at it and react to it," he said.
The letter has been signed by, among others, Lantos, Ileana Ros-Lehtinen, the ranking Republican on the foreign relations panel, and Gary Ackerman who is the chair of the house Middle East and South Asia sub-committee.
It contains a thinly veiled threat that if India continues with its relations with Iran it may jeopardise the landmark nuclear deal between the two countries agreed upon nearly two years ago.
The letter stresses that "the subject of India's strengthening relationship with Iran will inevitably be a factor" when the US Congress votes on a final 123 agreement that the two sides are currently negotiating.
"As strong proponents of closer ties between the United States and India, we are deeply concerned that the developments outlined in this letter have a significant potential to negatively affect relationship between the US and India in general and consideration by Congress of the 123 Agreement in particular," they said.
The letter objecting to New Delhi's ties with Tehran comes close on the heels of Foreign Secretary Shivshankar Menon's visit to Washington for talks aimed at putting the 123 negotiations back on track.
When asked a question on this issue, Menon had clarified that India's relations with Iran were not in contravention of the UNSC resolutions.
US Undersecretary of State for Political Affairs Nicholas Burns will be going to New Delhi later this month to clinch a final agreement, according to State Department spokesperson Sean McCormack.