US National Security Adviser John Bolton has been holding talks in Russia after it condemned US plans to pull out of a key nuclear treaty.
President Donald Trump said Russia had violated the Intermediate-Range Nuclear Forces (INF) pact. Russia denies this.
The treaty, which was signed in 1987, restricted US and Russian short- and medium-range nuclear missiles.
In Moscow, Mr Bolton was told that the US withdrawal would be a "serious blow" to the non-proliferation regime.
However, Russia's Security Council Secretary Nikolai Patrushev also said the Kremlin was "ready" to work with the US to remove "mutual" grievances over the INF.
Mr Bolton is reported to have been a key voice pushing for the withdrawal.
His talks with senior Russian officials were pre-planned, but now he will meet President Vladimir Putin too because of the latest developments.
As Mr Bolton began his visit, Moscow warned it would take steps to maintain the balance of nuclear power.
"We need to hear the American side's explanation on this issue," Kremlin spokesman Dmitry Peskov said. "Scrapping the treaty forces Russia to take steps for its own security."
What did the treaty do?
The INF treaty was signed by US President Ronald Reagan and Mikhail Gorbachev, the last Soviet leader, in the final years of the Cold War.
It banned ground-launched medium-range missiles, with a range of between 500 and 5,500km (310-3,400 miles) - both nuclear and conventional - effectively reducing the perceived threat to European nations from Soviet missiles.
It has stayed in effect for three decades but on Saturday President Trump said Russia had not adhered to the deal.
"So we're going to terminate the agreement and we're going to pull out," he said. "Russia has violated the agreement. They've been violating it for many years."
"And we're not going to let them violate a nuclear agreement and go out and do weapons [while] we're not allowed to."
The view from Russia
Sarah Rainsford, BBC Moscow correspondent
With one voice, Russian politicians, analysts and the state media are arguing that it is Donald Trump, not Russia, endangering global security by threatening to pull out of the landmark deal.
In fact, the treaty has its critics in Russia too. Some here consider it a "Cold War relic" - especially as China and others develop their missile systems, unhindered. President Vladimir Putin himself has threatened to abandon it in the past, accusing the US of non-compliance.
So President Trump may just be doing Moscow military hawks a favour, untying Russia's hands and allowing it to blame Washington for sparking a renewed arms race.
Today the Kremlin spokesman underlined that the US has so far taken no actual steps to pull out of the deal. But if it does, Dmitry Peskov warned, that will make the world a more dangerous place.
How is Russia suspected of breaking the treaty?
The announcement followed reports in the Guardian newspaper that Mr Bolton, in his capacity as national security adviser, had recommended withdrawal from the treaty.
He is not the first to suggest doing so - Barack Obama publicly accused Russia of violating the treaty in 2014 but ultimately chose not to pull the US out of the deal.
Mr Bolton will now face meetings with Foreign Minister Sergei Lavrov and other top officials.
Mr Putin is "seeking answers", according to a Kremlin spokesperson.
Russia has always denied any violations of the treaty but has accused Washington of violations in return.
On Sunday, one of the original signatories of the INF treaty, Mikhail Gorbachev, said a US withdrawal would reverse efforts made to achieve nuclear disarmament.
But the US insists the Russians have, in breach of the deal, developed a new medium-range missile called the Novator 9M729 - known to Nato as the SSC-8.
It would enable Russia to launch a nuclear strike at Nato countries at very short notice.
Withdrawal from the treaty is also seen as a counter-move to China, which has not signed up to the deal and can therefore develop such weapons at will.
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