The head of Ukraine's oil and gas monopoly Naftogaz has undergone emergency heart surgery and is now in intensive care.
According to Prime Minister Yulia Timoschenko the pressure put on Oleg Dubina during gas talks between Russia and Ukraine led to his health problems.
Oleg Dubina played a crucial role in these complicated negotiations.
Moscow stopped its gas supplies to Ukraine on January 1 because of unpaid debt. A week later Kiev blocked Russian transit gas supplies to Europe.
The two countries finally resolved the dispute on January 19 after intense negotiations.