Poroshenko's political party, European Solidarity, said the former president had scheduled only meetings in Poland and the United States and warned the SBU security service against becoming involved in politics.

Poroshenko was turned away at a border post on Friday.

The SBU said he had planned to meet Hungarian Prime Minister Viktor Orban, who maintains ties with Kremlin leader Vladimir Putin and opposes opening talks on European Union membership with Ukraine.

An SBU statement said Russia was preparing a series of "provocations" to discredit Ukraine among its foreign allies as the war against Russia stretches beyond 21 months.

It provided no evidence to support the allegations.

Of the possible meeting with Orban, the SBU said: "Russia planned to use this meeting (like other 'working meetings with ... representative of countries voicing pro-Russian narratives) in psychological operations against Ukraine."

The SBU said Orban "systematically holds an anti-Ukrainian position", was a "friend of Putin" and sought the removal of sanctions imposed on Moscow over its invasion of Ukraine.

Orban, who is open about his friendly ties with Putin, opposes the start of EU membership talks with Ukraine, to be considered at the bloc's forthcoming summit. He called this week for the creation instead of a "strategic partnership" with Kyiv.

Orban has frequently been at odds with Zelenskiy on several issues related to Ukraine's EU membership bid. European Solidarity, in its statement, called for dialogue with Orban who, it said, had a veto right over accession talks.

Under martial law, Ukrainian officials must secure approval to travel abroad. Parliament's deputy speaker, Oleksandr Korniyenko, said Poroshenko's permission had been cancelled after he had received a letter, which he could not comment on.

Zelenskiy's office has made no comment and Orban's office did not immediately respond to a request for comment.

Poroshenko, president from 2014 to 2019, accused Zelenskiy's administration on Friday of cancelling the permission and playing politics ahead of elections.

The dispute comes amid slowly growing tensions between government and opposition - mostly over internal matters such as budgets and appointments - in contrast to the near-total unity at the start of the conflict.

Zelenskiy and Poroshenko fought a bitter, often deeply personal battle in the 2019 presidential election, when Zelenskiy defeated the incumbent Poroshenko in a landslide.

Zelenskiy said last month that it was "not the time" to hold a presidential election, which under normal circumstances would be scheduled for March 2024 but is prohibited under martial law.

(Additional reporting by Boldizsar Gyori in Budapest, Editing by Ron Popeski and Daniel Wallis)

By Max Hunder