US President Barack Obama says Donald Trump's embrace of Russian President Vladimir Putin is unprecedented and he is troubled that other Republicans are supporting the Republican presidential candidate's positions on Russia.
"Mr Trump's continued flattery of Mr Putin and the way he appears and the degree to which he appears to model many of his policies and approaches to politics on Mr Putin is unprecedented in American politics," Mr Obama said at a White House news conference with Italian Prime Minister Matteo Renzi.
Saying "Mr Trump rarely surprises me these days", Obama said he was much more concerned to see support for the candidate's stance on Putin and Russia from Republican officials who historically had been anti-Russia.
Historically, Republican politicians have often taken or appeared to take a harder line on Russia's predecessor, the Soviet Union, the United States' Cold War foe.
Russia has been internationally condemned for its 2014 annexation of Crimea from Ukraine and its support of pro-Russian separatists in eastern Ukraine.
Mr Obama also urged Mr Trump to "stop whining" about the November 8 election being rigged, saying no serious person could suggest US elections could be manipulated because of their decentralised nature.
"I have never seen in my lifetime, or in modern political history, any presidential candidate trying to discredit the elections and the election process before votes have even taken place. It's unprecedented," the Democratic president said.
"I'd invite Mr Trump to stop whining, and go try to make his case to get votes."