Spain on Monday raised the issue of "disinformation and manipulation" emanating from Russia during the Catalan independence crisis, at a meeting of EU foreign and defence ministers.
Putin and Trump, who had their first face-to-face meeting in July to discuss allegations of Russian meddling in the 2016 US election, are both planning to attend this week's summit in the Vietnamese city of Danang.
President Donald Trump's son-in-law, Jared Kushner, a senior White House adviser, is under scrutiny by the Federal Bureau of Investigation in the Russia probe, the Washington Post and NBC News reported on Thursday.
The Republican leader of the House investigation into Russian interference in the US election announced on Thursday he was stepping aside after being criticised for compromising the probe in visits to the White House.
President-elect Donald Trump will reportedly name ExxonMobil chief Rex Tillerson as secretary of state despite mounting support for a congressional probe into Moscow's alleged interference in the US election.
US presidential candidate Hillary Clinton said the challenges facing Americans demand steady leadership and a collective spirit, contrasting her character with what she described as a dangerous and volatile Donald Trump.