President Donald Trump and Russian President Vladimir Putin shake hands as they hold a joint news conference after their meeting in Helsinki, Finland July 16, 2018.

President Vladimir Putin and U.S. President Donald Trump discussed a possible referendum in separatist-leaning eastern Ukraine during their Helsinki summit earlier this week, Russia's ambassador to the U.S. said Friday.

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

"This issue (of a referendum) was discussed," he said, adding without elaborating that Putin made "concrete proposals" to Trump on solutions for 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.

Trump tweeted that the two men discussed Ukraine, but has not mentioned a referendum or revealed specifics. The U.S. and Russia have been on opposing sides of the conflict in Ukraine, unleashed after a popular uprising against a pro-Russian president and Russia's annexation of Crimea in 2014.

Ukraine and European powers did not immediately comment Friday, but 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.

Putin has sought to ally with Trump at a time when U.S. ties to Europe are fraying, and both presidents are critical of the EU and NATO. German Chancellor Angela Merkel pushed back Friday at Trump's questioning of NATO's central tenet of collective defense.

Ambassador 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."

Antonov 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 U.S.-Russian cooperation on Syria's future.

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

Antonov said it's 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 hasn't responded yet to the proposal Trump made Thursday.

The Russian ambassador to Washington also denounced "anti-Russian anger" in the United States and the "severity" of the U.S. criticism of Trump's performance at the summit.

He reiterated denials of Russian meddling in the 2016 presidential election — interrupting a questioner to say "We didn't interfere!" He also reiterated denials of Russian involvement in the poisoning of former Russian spy Sergei Skripal in Britain.

Meanwhile, Antonov also pushed for the release of a gun rights activist accused of being a covert agent in the U.S., calling her arrest a "farce."

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

Butina, 29, denies wrongdoing, and the Russian Foreign Ministry started an online campaign for her release.

Dmitry Kozlov in Moscow and Geir Moulson in Berlin contributed.