Washington, United States: US President Donald Trump pledged Sunday to work "constructively" with Russia but ruled out an immediate easing of sanctions while the countries remain at odds over the conflicts in Syria and Ukraine.

In a series of tweets on his return from Europe, Trump said he had confronted his Russian counterpart Vladimir Putin over evidence from the US intelligence agencies that Moscow meddled in the 2016 election when the two leaders met for the first time in Germany on Friday.

And while he welcomed an agreement for the start of a ceasefire in Syria, Trump said it was too early to consider any easing of US sanctions on Russia "until the Ukrainian & Syrian problems are solved."

"I strongly pressed President Putin twice about Russian meddling in our election," Trump said of their meeting on the sidelines of the G20 summit. "He vehemently denied it. I've already given my opinion....."

Trump said he and Putin had talked about setting up what he called "an impenetrable cyber security unit" to prevent hacking in future elections, an idea that drew ridicule from members of his own Republican party.

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!"

- Friction over Syria -

Syria has been a particular source of friction between the two countries, as Russia is a close ally of President Bashar al-Assad. 

Moscow was furious when the Trump administration launched a cruise missile strike against Syrian forces in April, in retaliation for what Washington said was a chemical weapons attack by Assad's regime against civilians.

Moscow has warned that a program 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 US 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 Russians and pro-Russian rebels it blames for the fighting in Ukraine and the occupation of Crimea.

"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 president has previously equivocated over whether Russia did try to tilt the outcome of last November's election contest against Hillary Clinton in his favor, amid an investigation into whether members of Trump's campaign team actively colluded with Moscow.

- 'Strategic alliance' -

So his public assessment that Russia did meddle has triggered questions over whether his administration planned to bring in more sanctions.

Asked on Sunday whether new sanctions were in the pipeline, US Treasury Secretary Steve Mnuchin told ABC television: "We have sanctions that are already on the table and we expect to enforce those sanctions."

Mnuchin also insisted that Russia and the US could work together on cyber security, despite criticism that the two sides had diametrically opposing goals.

"What we want to make sure is that we coordinate with Russia, that we're focused on cybersecurity together, that we make sure that they never interfere in any democratic elections," he said.

"This is like any other strategic alliance, whether we're doing military exercises with our allies or anything else. This is about having capabilities to make sure we both fight cyber (crime) together, which I think is a very significant accomplishment for President Trump."

But senior Republican senators, including former presidential candidate John McCain, poured scorn on the idea.

Lindsey Graham, a member of the Senate's armed services commitee, said on NBC that the cyber idea was "not the dumbest idea I've ever heard, but it's pretty close."

Voice dripping with sarcasm, McCain told a CBS interviewer that he was "sure that Vladimir Putin could be of enormous assistance in that effort, since he's doing the hacking."

McCain also criticised the Trump administration for not putting together new sanctions specifically for the election meddling.

"So far they have not paid a single price for that," he said.

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