But reports on Tymoshenko's declining health did not provide specifics on what is ailing her.
Nina Karpachova, head of the human rights commission in the Ukrainian Parliament, said she made an unannounced visit to Tymoshenko Sunday at Lukyanivka jail in Kiev, National Radio Co. of Ukraine reported.
"(Tymoshenko's) condition is extremely grave. She could not get out of bed when talking to me. She needs examination and treatment outside the detention center," Karpachova said Monday.
The former prime minister has been held more than three months after being convicted of abusing her authority in a 2009 gas deal with Russia's Gazprom while serving as prime minister.
Tymoshenko, who has denied the charges, was sentenced to seven years in prison in October for abuse of office.
The verdict has been widely condemned.
The European Union and the United States have said Tymoshenko's sentence was a politically motivated and selective use of the judicial system. Russia has also criticized the sentence as politically motivated.
Tymoshenko was defeated last year by pro-Kremlin Viktor Yanukovych in Ukraine's presidential election.