Talks between the US and Russia about a US anti-missile system in Europe have ended acrimoniously and without any sign of progress.
The US goal was to make progress on some crucial strategic issues
The US rejected Russian appeals at the talks in Moscow to halt the scheme.
Russian Foreign Minister Sergei Lavrov said his country would take steps to neutralise the threat posed by the missile system if it went ahead.
He said it was based on a false assumption - that there was a nuclear threat from Iran.
US Defence Secretary Robert Gates said the shield system was not directed at Russia.
Mr Gates said he and US Secretary of State Condoleezza Rice had put several new ideas to the Russians but indicated that they had not yet been accepted.
"Our talks reflected the complex, multi-faceted relationship the US and Russia have," he said.
"We remain eager to be open and full partners with Russia in missile defence... We discussed a range of proposals we hope they will accept."
But Mr Lavrov said the proposals needed to be studied and that meanwhile Washington should halt work on the shield.
"We believe that to make the joint work of Russian and US experts most effective, plans on deploying [the missile defence system in Europe] should be frozen," he said.
Ms Rice said talks with Poland and the Czech Republic on deploying elements of the shield on their soil would continue.
"We will work during this time to address Russian concerns... We believe that we can address those concerns and we are prepared to do it," she said.
The US says it needs a missile defence system to counteract "rogue states" like Iran and North Korea.
The Kremlin has asked the US why it cannot instead use Russian-operated early warning radar in Azerbaijan.
Mr Gates said while that radar might be used, it was not capable of guiding interceptor missiles.
The BBC's diplomatic correspondent, Jonathan Marcus, says the news conference after the talks showed the underlying tensions between Washington and Moscow.
He says that this should have been an opportunity to work on the climate of US-Russia relations but the outcome may well have made things worse.
Earlier, Russian President Vladimir Putin signalled he would not support US plans and threatened to abandon a key nuclear missile treaty which he said was outdated.
President Putin said it would be difficult to remain part of the Intermediate Range Nuclear Forces (INF) treaty unless it was expanded to include more countries than just the US and Russia.
The reason, he said, was that other countries were developing these kinds of weapons systems - including those close to Russia's borders.
Analysts say President Putin's threat to withdraw from the treaty is yet another diplomatic move to put pressure on the Americans.
The treaty, which limits US and Russian short and medium range missiles, was signed 20 years ago and led to the elimination of almost 3,000 Russian and US missiles.
Russia has additionally threatened to leave the Treaty on Conventional Armed Forces in Europe if it is not ratified by all Nato nations.
The US-Russian talks also covered the Iranian nuclear issue.
Mr Lavrov criticised US sanctions and hints about using military force against Iran, which he said "contradict our collective efforts" to negotiate a solution.
US MISSILE DEFENCE: LONG RANGE THREAT PROTECTION
US wants to build defence system against possible missile attacks
Part of defences would be in Eastern Europe - which Russia opposes
Russia suggests US should use its Gabala, Azerbaijan base instead