Russian Vladimir Putin attends a meeting with Russian ambassadors to foreign countries in Moscow, Russia, Thursday, July 19, 2018. Putin says his first summit with U.S. President Donald Trump was "successful" and is accusing Trump's opponents in the U.S. of hampering any progress on the issues they discussed. (Sergei Karpukhin/Pool Photo via AP)

Russian Vladimir Putin attends a meeting with Russian ambassadors to foreign countries in Moscow, Russia, Thursday, July 19, 2018. Putin says his first summit with U.S. President Donald Trump was "successful" and is accusing Trump's opponents in the U.S. of hampering any progress on the issues they discussed. (Sergei Karpukhin/Pool Photo via AP)

MOSCOW (AP) — Russian President Vladimir Putin and U.S. President Donald Trump discussed the possibility of a referendum in separatist-leaning eastern Ukraine during their Helsinki summit, Russia’s ambassador to the U.S. said Friday.

However, the White House said later that it is not considering supporting the idea.

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

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“This issue (of a referendum) was discussed,” he said, adding that Putin made “concrete proposals” to Trump on solutions for the four-year, Russian-backed insurgency in eastern Ukraine, which has killed more than 10,000 people.

He did not elaborate on what Putin’s solutions would be.

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

Later Friday, U.S. National Security Council spokesman Garrett Marquis said “The administration is not considering supporting a referendum in eastern Ukraine.”

“To organize a so-called referendum in a part of Ukraine which is not under government control would have no legitimacy,” he said.

Trump tweeted that the two leaders had discussed Ukraine, but had not mentioned a referendum or revealed specifics of the talks with Putin.

The U.S. and Russia have been on opposing sides of the conflict in Ukraine, which was unleashed after a popular uprising in 2014 against Ukraine’s pro-Russian president and Russia’s annexation of Ukraine’s Crimean Peninsula a month later.

Ukraine’s government didn’t immediately respond to the developments but former Foreign Minister Konstantin Grishchenko said, “Kiev will never agree to a referendum in the Donbass initiated by Russia.”

“The Kremlin already used a referendum as justification for annexing Crimea. And Moscow is trying to use this theory and spread it to the Donbass,” he said on the 112.ua television network.

International powers did not recognize the Crimean referendum that preceded Russia’s annexation. Ukraine’s Donetsk and Luhansk regions also held referendums in 2014 that were not recognized by Ukraine’s government.

European powers are also expected to oppose 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 for the region.

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Putin has sought to become a Trump ally 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 the two leaders 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 the leaders’ first summit before jumping too fast into a new one.

″(But) Russia was always open to such proposals. We are ready for discussions on this subject,” he said.

The Kremlin hasn’t responded yet to the proposal Trump that made Thursday.

The Russian ambassador to Washington also denounced what he saw as “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 U.S. 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 and his daughter in Britain.

Antonov also pushed for the release of a Russian gun rights activist who has been 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 Trump’s election as president.

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

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Yuras Karmanau in Minsk, Dmitry Kozlov in Moscow and Geir Moulson in Berlin contributed.