Four US senators frequently critical of the White House on national security issues pressed the Pentagon on Monday to say whether Russian entities were helping Iran develop ballistic missiles.

Republican Senators Jon Kyl, Mark Kirk, John Barrasso, and Jim DeMint also asked for access to draft documents in Russo-US talks on the deployment of a proposed US missile defense shield in Europe.

"We renew our request for any draft materials you are discussing with the Russians," including a draft Missile Defense Cooperation Agreement (MDCA) and Defense Technology Cooperation Agreement (DTCA), they said.

The message came in a letter to Principal Deputy US Undersecretary of Defense for Policy James Miller, who met with the lawmakers on May 12 to discuss their "concerns" about the missile defense talks between Washington and Moscow.

Miller offered "take back" to the Pentagon the request for access to the documents, according to the letter, a copy of which was obtained by AFP.

The senators also pressed Miller on a past US intelligence assessment they quoted in the letter as saying that "individual Russian entities continue to provide assistance to Iran's ballistic missile programs."

The assessment also said that Russian help "along with assistance from entities in China and North Korea, has helped Iran move toward self-sufficiency in the production of ballistic missiles," according to the letter.

"In 2010, the Office of the Director of National Intelligence reaffirmed that assessment. Do you agree with this assessment?" they wrote Miller.

The senators said they hoped for a briefing "as soon as possible" to address their worries about the security of sensitive US data and systems in any final agreement on cooperating with Moscow.

And they paraphrased a top US Missile Defense Agency official as saying that deploying a radar "in either Georgia or Armenia would have significant advantages for the missile defense of the United States" and asked for more details.