On Thursday, Trump expressed gratitude for Putin’s decision to reducing the number of US diplomats in Russia by 775, saying the decision would help the US government save money.

"The President was being sarcastic," Sanders said as quoted by NBC News. "We take seriously Moscow's unwarranted actions against our personnel and diplomatic properties, and we are exploring our options."

The remarks sparked immediate criticism from some lawmakers, who claimed Trump's comments are an insult to US embassy personnel.

Moscow announced in the end of July that the diplomatic presence of the United States in Russia would be cut by 755 people to 455, the same number of diplomatic personnel that Russia currently has in the United States.

The Kremlin said the move came in response to the 2016 ouster of 35 Russian diplomats, as well as new sanctions passed by the US Congress in July.