FILE PHOTO: Russian opposition leader Alexei Navalny attends a hearing after being detained at the protest against corruption and demanding the resignation of Prime Minister Dmitry Medvedev, at the Tverskoi court in Moscow, Russia March 27, 2017. REUTERS/Tatyana Makeyeva/File Photo

MOSCOW (Reuters) - Kremlin critic Alexei Navalny said on Thursday that authorities in Russia were stopping him going abroad to seek urgent medical treatment after an attack that left him mostly blind in one eye.

Navalny said the authorities had unexpectedly issued him with an international passport of the kind Russians need to travel abroad after refusing to do so for the last five years.

That had given him the impression he could fly to a foreign clinic to get treatment for a chemical burn that has robbed him of most of his sight in his right eye after an assailant threw green liquid in his face on April 27, he said.

But he said a senior official from the country’s prison service had phoned his lawyer on Thursday to say that Navalny could not travel abroad because of an embezzlement conviction against him.

Navalny says the conviction, which came with a 5-year suspended prison sentence, was politically-motivated and aimed at sabotaging his plans to run for president in March next year.

“So they gave me a foreign travel passport, but have banned me from traveling,” Navalny said on social media. “Why did they give it (the passport) to me then? To use it to wrap up fish?”

Navalny said he had only 15 percent vision in his right eye, but there was a chance it could get better with treatment.

Navalny, who a senior election official said on Wednesday was ineligible to run for president, organized the biggest anti-government protests in years in March, and has become the most prominent opposition challenger to Vladimir Putin, who is expected to run for what would be a fourth presidential term next year.