US President Donald Trump has thanked President Vladimir Putin for expelling more than 750 American diplomatic staff from Russia.

The comments were immediately criticised by current and former US officials.

"I'm very thankful that he let go of a large number of people because now we have a smaller payroll," said Trump on Thursday, breaking nearly two weeks of silence on Putin's July 30 order cutting US embassy and consulate staff by nearly two-thirds.

"There's no real reason for them to go back … We're going to save a lot of money."

The State Department had no immediate reaction to the comments Trump made to reporters while on vacation at his golf club in Bedminster, New Jersey.

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Congressional committees and a special counsel are investigating the conclusions of US intelligence agencies that Russia interfered in the 2016 election campaign by hacking and other methods to help Trump.

They are also looking into possible collusion between the campaign and Russian officials. Moscow has repeatedly denied meddling in the election and Trump denies any campaign collusion.

Putin, reacting to new sanctions imposed by the US Congress and reluctantly signed into law by Trump, ordered Washington to cut 755 of its 1,200 embassy and consulate staff by September. Many of those affected likely will be local Russian staffers.

During his campaign and since becoming president, Trump has consistently called for better ties with Russia, declined to criticise Putin, and refused to unequivocally embrace the conclusions of the intelligence agencies.

'Grotesque'

Trump's remarks on Thursday were immediately denounced by current and former US officials who have served both Republican and Democratic administrations.

Nicholas Burns, the State Department's third-ranking official under former president George W Bush, called Trump's comments "grotesque".

"If he was joking, he should know better," said Burns, now a professor at Harvard University's John F Kennedy School of Government.

"If he wasn't, it's unprecedented. A president has never defended the expulsion of our diplomats."

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The State Department was "horrified and rattled" by Trump’s remarks, said a veteran US diplomat who has served in Russia, speaking on condition of anonymity.

And Heather Conley, formerly a top State Department official dealing with European affairs, said the expulsions of hundreds of people from an important US embassy is extraordinary.

"It is very difficult to see how the president could view these expulsions as a 'positive' development in any form," Conley said.