Serbia's President Aleksandar Vucic was admitted to a military hospital in Belgrade on Saturday after cutting short a visit to the United States, his office said, without giving a reason.
Vucic's hospitalisation comes as he faces a widespread protest movement over a deadly accident at a train station in the northern city of Novi Sad.
Vucic, who according to Serbian media reports cut short his trip to visit US President Donald Trump in Florida after feeling unwell, "was admitted to the Military Medical Academy immediately after landing" in Belgrade, his office said.
It said it would release more detailed information on Vucic's health "in good time".
Vucic had said he was to meet Trump twice, but cut short his trip after feeling unwell on consultation with a doctor, Serbian broadcaster RTS reported.
The Serbian president, elected in 2017 following three years as prime minister, was hospitalised in 2019 for heart issues.
Vucic is facing down a wave of demonstrations sparked by the roof collapse at the Novi Sad train station in November 2204, which killed 16 people.
Many in the Balkan nation blame the collapse on chronic corruption and a lack of oversight over construction projects.
Opposition figures said Vucic's trip was a failure since he did not meet with Trump as planned.
"He's covering up his fiasco with a story about a sudden health problem forcing him to rush back to Belgrade. There are no medicines in pharmacies in America," quipped Zdravko Ponos, leader of the centrist Srce party and a fierce critic of Vucic, and who ran against him in the 2022 presidential vote.
The announcement also casts doubt over Vucic's promised visit to Serbia's traditional ally Russia for celebrations marking the 80th anniversary of the end of World War II in Europe on May 9.
Vucic said Thursday that he would keep his word to Vladimir Putin and join around 20 world leaders travelling to Moscow for the occasion, commemorated with great pomp and an army parade on Red Square.
But the trip to visit Putin would have risked the wrath of the European Union, which Serbia wishes to join, and which has sanctioned Russia over its invasion of Ukraine.
Ukrainian President Volodymyr Zelensky said his country would not take responsibility for the security of foreign leaders in Russia.
Russia was quick to jump on the Ukrainian leader's comments, branding them "a direct threat" to the celebrations.
