"Today I met with the Russian delegation. Tomorrow there will be meetings with, among others, the Chinese delegation," the high-ranking source told reporters on site.
The preparatory talks are being held after the nuclear arms agreement between the US and Russia - the New START agreement - expired.
The US also says that talks are underway with Britain and France.
The five countries are recognized as nuclear powers under international law. Russian leader Vladimir Putin says he is willing to temporarily extend the START treaty by a year if the US agrees to concessions.
US President Donald Trump is demanding that China also be included in a new agreement, which Beijing opposes.
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