KIEV, Ukraine - Ukrainian President Viktor Yanukovych is renewing calls for funding to build a new shelter around the Chernobyl nuclear reactor on the accident's 25th anniversary.

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Ukraine still needs to raise some $300 million for the project after an international donors conference held here earlier this month.

Yanukovych said in a statement early Tuesday that "no nation ... can overcome the consequences of a catastrophe of such a scale by itself."

Ukrainians began marking the anniversary of the world's worst nuclear disaster in the wee hours of Tuesday. In a memorial service in Kiev led by Russian Orthodox Patriarch Kirill, priests sang solemn hymns, Ukrainians lit thin wax candles and a bell tolled 25 times for the years that have passed since the disaster.