Russian president Vladimir Putin and his US counterpart Donald Trump discussed a possible referendum in separatist-leaning eastern Ukraine during their Helsinki summit earlier this week, Russia’s ambassador to the US said.

Anatoly Antonov revealed the discussions amid confusion and concern in the US about what the two presidents agreed behind closed doors.

Mr Antonov said: “This issue (of a referendum) was discussed,” adding that Mr Putin made “concrete proposals” to the US president on solutions to the four-year Ukraine conflict, which has killed more than 10,000 people.

The move may be seen as an effort to sidestep European peace efforts for Ukraine and increase pressure on the Ukrainian government in its protracted conflict with pro-Russian separatists in the Donbass region.

Mr Trump tweeted that the two men had discussed Ukraine, but has not mentioned a referendum or revealed any specifics.

The US and Russia have been on opposing sides of the conflict in Ukraine, which unleashed after a popular uprising against a pro-Russian president and Russia’s annexation of Crimea in 2014.

Ukraine and European powers are not expected to support a referendum in the Donbass, where pro-Russian separatists hold sway.

They have been committed to a 2015 peace deal signed in the Belarusian capital of Minsk that has helped reduce the fighting, but failed to find a lasting political solution.

Mr Putin has sought to ally with Mr Trump at a time when US ties to Europe are fraying, and both presidents are critical of the EU and Nato.

German chancellor Angela Merkel has criticised Mr Trump’s repeated questioning of Nato’s central tenet of collective defence.

Mr Antonov called Monday’s summit in Helsinki a “key event” in international politics, and laughed off suggestions that the two men made any “secret deals”.

Donald Trump and Vladimir Putin shake hands in Helsinki
Donald Trump and Vladimir Putin shake hands in Helsinki (Pablo Martinez Monsivais/AP)

He insisted that diplomatic discussions should remain discreet in order to be effective, but gave a few details of their discussions on arms control and said the summit notably made progress on US-Russian co-operation on Syria’s future.

The ambassador also said Moscow is ready to discuss a possible visit by Mr Putin to Washington after a surprise invitation from Mr Trump.

Mr Antonov said it is important to “deal with the results” of their first summit before jumping too fast into a new one, but that “Russia was always open to such proposals. We are ready for discussions on this subject”.

The Kremlin has the final say, but has not responded yet to the proposal Mr Trump made on Thursday.

News of the invite caught many in Washington off-guard.

Donald Trump and Vladimir Putin
Mr Trump and Mr Putin held a historic summit this week (AP)

“Say that again?” national intelligence director Dan Coats responded, when informed of the invitation during an appearance at the Aspen Security Forum in Colorado.

“OK,” he continued, pausing for a deep breath. “That’s going to be special.”

The Russian ambassador to Washington also denounced “anti-Russian anger” in the United States and the “severity” of the US criticism of Mr Trump’s performance at the summit.

He reiterated denials of Russian meddling in the 2016 presidential election – interrupting a questioner to declare: “We didn’t interfere!”

Mr Antonov also reiterated denials of Russian involvement in the poisoning of former Russian spy Sergei Skripal in the UK.

Meanwhile, the ambassador also pushed for the release of a gun rights activist accused of being a covert agent in the US, calling her arrest a “farce”.

US federal prosecutors accused Maria Butina this week of being a covert Russian agent and working to infiltrate US political organisations, including the National Rifle Association, before and after Donald Trump’s election as president.

Butina, 29, denies wrongdoing, and the Russian foreign ministry has started an online campaign for her release.