US President Donald Trump has tweeted that "it is time to move forward in working constructively with Russia," citing a ceasefire negotiated for southwest Syria as proof that this is possible.

Donald Trump met with Russian President Vladimir Putin for more than two hours in Hamburg on Friday, the first day of the G20 summit.

"We negotiated a ceasefire in parts of Syria which will save lives. Now it is time to move forward in working constructively with Russia!" Trump wrote on Twitter.

Trump also tweeted that he and Russian President Vladimir Putin had discussed forming an impenetrable cybersecurity unit to prevent election hacking.