The Ukrainian presidency raised new doubts on Monday over the deal that ended its gas standoff with Russia, saying the accord could be declared invalid if it was signed under pressure.
President Viktor Yushchenko's office has repeatedly protested that the deal agreed last week by his arch-rival Prime Minister Yulia Tymoshenko and her Russian counterpart Vladimir Putin was a sellout for Ukraine.
The standoff between Ukrainian state gas company Naftogaz and Russian energy giant Gazprom resulted in several European countries being deprived of gas supplies for a two weeks amid freezing winter temperatures.
"The agreement between Naftogaz and Gazprom could be declared invalid if it was concluded under pressure," said Bogdan Sokolovsky, Yushchenko's special representative on energy security.
His comments came after it emerged that Naftogaz's chief executive Oleg Dubina, who had been in the thick of negotiations to end the crisis, was hospitalised last week with heart trouble.
"There are reasons to doubt that he was able to sign the documents knowingly and of his own free will," Sokolovsky said in a statement.
He said there were "signs of pressure" on the Naftogaz chief but that these still had to be verified.
Tymoshenko said at the weekend that contradictory orders issued by herself and Yushchenko during the gas crisis had proved so stressful to Dubina that they caused heart problems.
The presidency said that Dubina had been hospitalised on Thursday and will remain in hospital for two to three weeks.
Tymoshenko and Yushchenko were allies in the 2004 Orange Revolution that brought a pro-Western government to power in Ukraine but have since fallen out in an acrimonious power struggle marked by increasingly angry accusations.