Mr Medvedev said linking Iran and the missile shield would not be productive
US President Barack Obama has written to his Russian counterpart, Dmitry Medvedev, suggesting co-operation in blocking Iranian missile plans.
Mr Obama offered to suspend plans for a US missile defence shield, if Moscow backed efforts to stop Iran developing long-range missiles, officials said.
The letter, delivered last month, was a response to an earlier Russian letter.
President Medvedev said on Tuesday that he was ready to co-operate on Iran, but had received no offer of a trade-off.
"If we are to speak about some sort of exchange, the question has not been presented in such a way, because it would not be productive," he told reporters in Madrid after meeting Spanish PM Jose Luis Rodriguez Zapatero.
"Our American partners are ready to discuss this problem, and that's already positive," he added.
"Several months ago, we were hearing different signals."
Mr Medvedev said he hoped that the situation would be different under the Obama administration, but stressed that "no-one is linking these issues to some kind of trade-offs, particularly on the Iranian issue".
"We are already working in close contact with our US counterparts on the Iranian nuclear issue," he said.
A senior US official told the BBC that a letter had been sent "a few weeks ago" to the Kremlin outlining a possible initiative involving the missile shield and repeating US concerns about Russia's lack of support for Western efforts to put pressure on Iran over its long-range missile programme.
The official said the letter was hand-delivered, although it was not clear by whom.
Earlier on Tuesday, the New York Times newspaper quoted a senior Obama administration as saying: "It's almost saying to them, put up or shut up.
"It's not that the Russians get to say, 'We'll try and therefore you have to suspend.' It says the threat has to go away."
The paper added that "the letter was intended to give Moscow an incentive to join the United States in a common front against Iran".
The US says its planned missile defence system in Europe is intended to destroy incoming ballistic missiles fired by "rogue states", such as North Korea and Iran. However, Moscow says they could directly threaten its own defences.
The Iranian military says its missiles have a range of 2,000km (1,240 miles), which would mean they could potentially hit targets in Greece, Bulgaria or Romania, all Nato members.
Establishing a link
BBC diplomatic correspondent Jonathan Marcus says the US is seeking to establish a link between its own missile defence plans and Iran's development of ever longer-range missiles potentially capable of delivering a nuclear warhead.
US Defence Secretary Robert Gates made this linkage explicit in comments at a Nato summit in Poland last month, noting that he had "told the Russians a year ago that if there were no Iranian missile programme there would be no need for the US missile sites".
But, our correspondent says, it is not clear how Russia could limit Iran's missile programme, which rests largely upon a close relationship with North Korea.
There has been some Russian help in the past - from individuals or companies - but little to suggest any official assistance from Moscow, he adds.
Our correspondent says Washington may want Moscow to back a tougher sanctions regime against Tehran.
At present, Iran is a market for Russian weaponry, airliners and civil nuclear technology. A Russian-built reactor is expected to begin operation in Iran later this year.
Indeed, Moscow may feel it has stronger cards in its hand than Washington, our correspondent adds.
It controls vital land and air access routes to Afghanistan, which analysts believe could become crucial as existing supply lines for US and Nato forces through Pakistan come increasingly under attack.