Tymoshenko's defense teams said the former prime minister was staging a hunger strike to protest her treatment in prison and the conditions in Ukraine. She's reportedly bedridden because of back complications and her lawyer said she was roughed up during medical treatment last week.
The Russian Foreign Ministry, in a statement, said it was concerned about the reports.
"Moscow has treated with concern media reports about the deteriorating health of jailed ex-Ukrainian Prime Minister Yulia Tymoshenko, who urgently needs medical treatment," state-run news agency RIA Novosti quoted the statement as saying.
Tymoshenko is serving a seven-year prison sentence at a penal colony in Ukraine following a conviction on charges she abused her authority when she helped broker a 2009 natural gas deal with Russian energy company Gazprom.
Moscow called on Ukrainian authorities to "take all measures" to ensure Tymoshenko's rights are respected.
Tymoshenko denies the charges against her. Her supporters have said the charges were politically motivated.