KIEV - Ukrainian President Viktor Yushchenko on Thursday urged Russian President Dmitry Medvedev to change an agreement on supplies of Russian natural gas whose terms he said were too onerous for the Ukrainian economy.

In a move that appeared designed to embarrass his political rival, Prime Minister Yulia Tymoshenko, as she prepared for gas talks with Moscow, Yushchenko said an existing agreement on gas deliveries to Ukraine needed urgent revision.

In an open letter to the Kremlin leader, text of which was published on his website, Yushchenko said: Keeping the contracts unchanged ... will create potential threats specifically to the reliability of supplies of gas to Ukraine and its transit to other European states.

It is obvious that such a development of events is not welcome either for Ukraine or for Russia and the European Union, he said.

Tymoshenko was due later on Thursday to meet Russian Prime Minister Vladimir Putin in Yalta, southern Ukraine, for talks which analysts will monitor for signs of whether there could be another end-of-year dispute affecting Russian gas supplies across Ukraine to Europe.

The agreement referred to by Yushchenko was concluded between Ukraine's Naftogaz and the Russian gas monopoly Gazprom in January which ended a gas dispute between the two countries that affected supplies to Europe.

Russian gas supplies to Europe via Ukraine -- a route that supplies a fifth of Europe's gas -- were halted for more than two weeks at the start of the year due to that quarrel over prices and terms.

Millions in southern Europe were left without heating.

Eighty percent of exported Russian gas transits Ukraine to European markets.

Tymoshenko, who is running against Yushchenko in a presidential election on Jan. 17, says the gas agreement which she brokered with Putin is a good one and does not need to be revised. (Writing by Richard Balmforth; Editing by Janet Lawrence)

Russia Ukraine Dmitry Medvedev Vladimir putin