KIEV, Ukraine  —  Hackers from several countries launched a massive attack and temporarily disabled the Web site of Ukraine's Western-leaning President Viktor Yushchenko, his office said Tuesday.

A Russian nationalist group claimed responsibility.

The attacks from servers in Russia, Britain, Kazakhstan, the United States, Israel and Ukraine began Sunday night and continued through Tuesday afternoon, the presidential press service told The Associated Press.

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Over 18,000 attacks have been carried out, temporarily blocking access to the site. The Web page could not be accessed Tuesday night.

A radical Russian nationalist youth group, the Eurasian Youth Movement, claimed responsibility for the attacks in their blog, saying it was their retaliation for Yushchenko's office's alleged attack of their on Web site, which had been disabled.

The group called Yushchenko's government a "fascist regime" and accused it of attacking the organization's Moscow office. Yushchenko's office denied the claims.

The Eurasian Youth Movement is strongly critical of the West and opposes what it calls a U.S. encroachment on Russia's traditional sphere of influence.

The group has opposed Yushchenko's campaign to bring Ukraine into the European Union and NATO, considering the former Soviet republic part of Russia's area of influence.

Its leader, Alexander Dugin, has been barred from entering Ukraine.

Earlier this month, the group claimed responsibility for desecrating a monument to Ukraine's independence erected on top of the country's highest mountain — an act that drew condemnation from Ukrainians.