Russia on Wednesday suspended a three-year-old agreement with the United States on cooperation on nuclear and energy-related research — the latest move signifying a deteriorating bilateral relationship.

"It's frankly concerning," a senior U.S. official told VOA on condition he not be named.

Russian President Vladimir Putin on Monday also signed a decree suspending an agreement with the U.S. on disposing weapons-grade plutonium, citing "unfriendly actions" by Washington — a reference to U.S.-led sanctions on Moscow for its actions in Ukraine.

Nullifying the plutonium pact is "a real tragedy because these are areas where we have successfully cooperated in the past," State Department spokesman Mark Toner said during Wednesday's daily briefing.

The United States has halted Syria cease-fire direct talks with Russia amid continuing military attacks against civilian targets, with the U.S. administration declaring its patience at an end with Moscow. However, Secretary of State John Kerry and Russian Foreign Minister Sergei Lavrov spoke again Wednesday about Syria.

Diplomats in both Washington and Moscow say the United States requested the call, which also covered the situations in Ukraine and North Korea.

俄罗斯周三暂停了同美国在核能以及与能源相关研究的一个三年合作协议，这一最新举动标志着两国关系的继续恶化。

一位不愿意透露姓名的美国资深官员对美国之音说：“坦率地讲，这一情况令人担忧。”

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