President Dmitry Medvedev said Tuesday that there would be no trade-off on Iran's nuclear program and U.S. plans to set up a missile defense shield in Central Europe.

"No one sets conditions on these issues with trade-offs, especially on the Iranian problem," Medvedev said at a news conference in Madrid, where he was on a state visit. "In any case, we are working closely with our American colleagues on Iran's nuclear program."

Medvedev was responding to reporters' questions about a secret letter delivered three weeks ago from U.S. President Barack Obama.

The New York Times reported Tuesday that Obama suggested in the letter that the United States would back off from plans to place interceptor missiles in Poland and a radar station in the Czech Republic if Russia helped convince Iran to abandon its ambitions to develop a nuclear weapon.

But Obama said late Tuesday in Washington that the report did not "accurately characterize the letter," Reuters reported.

The U.S. State Department told The Moscow Times that the letter expressed Obama's readiness to discuss both missile defense and Iran with Russia but did not link the two.

"We are ready to consult with our NATO allies and Russia … [and] explore the possibilities of cooperation with Russia on configuration of the anti-missile defense system, using assets we have and we may develop" in order to eliminate threats to the United States, its allies and Russia, State Department spokesman Darby Holladay said by telephone from Washington.

He stressed that the Obama administration would discuss the missile defense options with the governments of Poland and the Czech Republic.

Medvedev's spokeswoman Natalya Timakova confirmed the existence of the letter to journalists in Madrid, and she stressed that it did not contain any mutually binding proposals.

Medvedev welcomed the letter as a sign that the new U.S. administration was prepared to discuss one of the main irritants in relations between the two countries.

"It is good already because several months ago we received a different signal: The decision has been made, there is nothing to talk about, we will do everything as decided," he said, RIA-Novosti reported.

On Nov. 5, the day Obama won the presidency, Medvedev threatened to place missiles on Russia's border with Poland if Obama followed through with plans by the administration of former U.S. President George W. Bush to station components of the missile defense shield so close to Russia's borders.

Senior Russian officials have said the missile defense plans intended to undercut Russia's capacity for a retaliatory strike in case of a nuclear attack against the country. Bush officials maintained that the Central European sites were needed to intercept possible missile attacks against the United States and its Western European allies from Iran.

The United States has long sought for Russia, which last week oversaw the test run of a Russian-built nuclear power plant in Iran and has provided the Islamic republic with short-range anti-aircraft missile systems, to exert some influence over Iran.

Russian officials have repeatedly said Moscow opposes a nuclear-armed Iran but at the same time have opposed stricter U.S.-backed international sanctions against Tehran.

The Obama letter was first reported Monday by Kommersant, which cited Washington sources. The New York Times published a front-page report Tuesday, citing unidentified Obama officials. It said the letter was hand-delivered by Undersecretary of State William Burns, a former U.S. ambassador to Moscow, and that Medvedev has not responded to Obama.

The two presidents are expected to meet in London on April 2.

Obama's position on the missile defense shield was formulated during his presidential campaign last year. He has said the effectiveness and affordability of the whole system needs to be proved before he decides to proceed with it. He has also said that he was ready to start direct negotiations with Iran. Iran and the United States officially cut ties in 1981.

Foreign Minister Sergei Lavrov on Monday urged the United States to restore diplomatic relations with Iran. "This would be an important element in stabilizing the situation in the region," he said.

Lavrov is set to discuss arms control — another abrasive issue of the U.S.-Russian relations — with U.S. Secretary of State Hillary Clinton in Geneva on Friday. Both sides have indicated that they are prepared to cut the number of their nuclear warheads, but Russia — facing the rapid aging of its nuclear deterrent infrastructure — is also seeking limits on delivery means, such as missiles, bombers and submarines.