He had been ill, and he spent eight days in hospital last month
Czech President Milos Zeman was receiving treatment in an intensive care unit on Sunday, creating uncertainty at a time when he is due to lead political talks about forming a new government after a parliamentary election.
Director Miroslav Zavoral of the Central Military Hospital in Prague said Zeman, 77, was admitted due to complications related to an undisclosed chronic condition.
"We know the diagnosis precisely, which allows us to target treatment," Zavoral said, adding that he did not have the president's approval to disclose details of the diagnosis.
Zavoral did not say how serious the president's condition was.
In the Czech Republic, the president leads talks with party leaders after elections to find a workable majority. Under the constitution, his authority to appoint the prime minister passes to the speaker of the lower house if the presidential position is vacated.
Reporters saw an ambulance leaving the presidential chateau at Lany, west of Prague, soon after Zeman met Prime Minister Andrej Babis.
The presidential office had previously said Zeman had been ill, and he spent eight days in hospital last month. The presidential office said at the time he had no life-threatening condition but was dehydrated and slightly exhausted.
Babis' centrist ANO party narrowly lost the parliamentary election held on Friday and Saturday to the centre-right Together coalition and appears to have no chance of forming a ruling majority.
Together and the liberal Pirates/Mayors coalition between them won 108 seats in the 200-seat lower house and said they intended to form a government.
However, while Babis has conceded that Together won more votes as a coalition, he did not signal a move into opposition, saying "If the president authorises me to do so, I will lead talks on forming a cabinet".
Forming govt may take months
Under the constitution, the president can appoint anyone as prime minister and instruct them to nominate a cabinet. A new cabinet must face a vote of confidence in the lower house within a month of its appointment.
Zeman had said prior to the election that he would appoint the leader of the biggest winning individual party, not a coalition, to try to form a government.
This would be Babis, since ANO won the most votes of any party.
Zeman, an ally of Babis, has made no comment on his next steps since the election results.
Forming a government usually takes weeks or months, and no appointments are possible before the new lower house convenes, some time within a month after the election.
Zeman has been using a wheelchair due to neuropathy in his legs since earlier this year.
He cancelled a trip to the polling station on Friday, instead casting a ballot to a box brought to him by election officials, without media access.
The public Czech Radio and newspaper denikn.cz cited seven sources as saying last week that the president had suffered from ascites, or liquid collecting in the abdomen, during his previous hospital stay. The president's spokesman then declined to comment on the report.