Russia and the United States gave mixed messages on Friday about whether their leaders will meet on the sidelines of a regional summit in Vietnam, with the White House ruling out an encounter but the Kremlin saying it could still happen.

Relations between Moscow and Washington have plummeted as a US probe has accused Trump's former campaign aides of secretly meeting Kremlin-connected officials — a charge Moscow denies.

Both leaders are attending the Asia Pacific Economic Cooperation (APEC) forum in the city of Danang and there have been conflicting indications in recent days on whether a face-to-face was on the cards.

The issue appeared to be finally settled shortly before Trump's touchdown in Danang on Friday afternoon when press secretary Sarah Huckabee Sanders briefed reporters on board Air Force One that there was not enough time for the pair to meet.

"There was never a meeting confirmed, and there will not be one that takes place due to scheduling conflicts on both sides," she said.

But Putin's team said a tete-a-tete could still take place.

"There is contradictory information coming from our American colleagues," Putin's spokesman Dmitry Peskov told reporters after Sanders' comments. "In any case, a meeting will take place on the sidelines."

Peskov's comments could indicate plans for Trump and Putin to share a handshake or brief conversation during one of the mass gatherings of leaders in Danang, rather than hold a full bilateral meeting.

Both Moscow and Trump had previously suggested that talks were likely to take place.

On Sunday, the US president said "it's expected we'll meet with Putin" to discuss North Korea's nuclear ambitions.

Four days later, Kremlin aide Yury Ushakov told Russian news agencies that the two would hold a meeting in Vietnam with the time and place still to be determined.

But hours later US Secretary of State Rex Tillerson said no decision had been made on talks, which would be the first between the two men since they met at the G20 summit in Germany in July.

Trump's relationship with Moscow has haunted the first year of his administration.

His former campaign chairman Paul Manafort and an associate are under house arrest on charges including conspiracy to launder money, linked to the investigation by special counsel Robert Mueller into allegations that the campaign colluded with Russia.

Russia has denied allegations of interfering in the US election last year that brought Trump to power.