President Viktor Yanukovych said Ukraine was no longer alone in combating the consequences of the Chernobyl nuclear disaster, as memorial services marking the accident's 25th anniversary began early Tuesday.

"Ukraine has been almost alone mitigating the Chernobyl disaster. But now we are not alone anymore," the president said in a national address early Tuesday.

A Chernobyl Fund of 550 million euros (737 million U.S. dollars) was set up at a donors conference held earlier this month. It will be used to build a new sarcophagus over the destroyed No. 4 reactor.

Activists lit 25 candles at Independence Square in Kiev at around 1 a.m., around the time of the blast on April 26, 1986 that spewed a cloud of radiactive materials over much of Europe. The infected area reached more than 50,000 square km in Ukraine alone.

Memorial services will be held across Ukraine during the day attended by many senior government officials and representatives of non-government organizations.

Russian President Dmitry Medvedev will also visit Kiev to discuss nuclear safety issues with the Ukrainian president.