HAMBURG (Reuters, AFP) - US President Donald Trump said in a tweet on Sunday (July 9) that he discussed forming a cyber security unit to guard against election hacking with Russian President Vladimir Putin.

Tweeting after his first meeting with Putin on Saturday, Trump said now was the time to work constructively with Moscow.

"Putin & I discussed forming an impenetrable Cyber Security unit so that election hacking, & many other negative things, will be guarded and safe," he said following their talks at the G20 summit in Hamburg, Germany. He did not give details.

Trump said he had raised allegations of Russian interference in the 2016 US presidential election.

"I strongly pressed President Putin twice about Russian meddling in our election. He vehemently denied it. I've already given my opinion....."

He also said the two men had discussed the implementation of a ceasefire in Syria which began on Sunday, saying "it will save lives." "Now it is time to move forward in working constructively with Russia!"

Syria has been a particular source of friction between the two countries, as Russia is a close ally of President Bashar al-Assad. Moscow has warned that a programme of sanctions imposed by the US, which was tightened last month, threatens their whole relationship.

Trump's predecessor Barack Obama ordered the seizure of two Russian diplomatic compounds in the United States last December after accusing Russia of trying to influence the outcome of the 2016 presidential election. And last month, the United States added 38 individuals and entities to its sanctions list targeting the Russians and pro-Russian rebels it blames for the fighting in Ukraine.

"Sanctions were not discussed at my meeting with President Putin. Nothing will be done until the Ukrainian & Syrian problems are solved," said Trump.

The US and Russian sides have issued sharply conflicting accounts of the meeting, with Putin saying on Saturday that Trump had been "satisfied" by his denials of any Russian interference in the polls.

Nikki Haley, the US ambassador to the United Nations, said the Russian denials had been expected but cut no ice.

"This is Russia trying to save face," she told CNN. "And they can't. They can't.

"Everybody knows that Russia meddled in our elections."