"If Ukraine has this ambition, my answer is NATO has an open door policy," De Hoop Scheffer told a press conference in Japan, the final destination in a nine-day tour that also took him to Australia and New Zealand.

"If Ukraine voices ambition to become a NATO member, NATO would take that ambition very seriously," he said.

"NATO is a performance-based organization. Ukraine is a distinctive partner of NATO. We will soon discuss this with Foreign Minister (Boris) Tarasyuk of Ukraine," he said.

Pro-Western President Viktor Yushchenko has urged both NATO and the European Union to open their doors to Ukraine to counter-balance the strong influence that Russia has long held over his country.

However, Yushchenko has also been cautious about a sudden move, telling AFP in a February interview that Ukrainian society was not ready to be part of an alliance vilified for decades by Ukraine's leaders as an instrument of US imperialism.

A February opinion survey in Ukraine showed nearly half -- 48 percent -- of voters would reject membership in the North Atlantic Treaty Organization, with only 15 percent in favor and 36 percent undecided.

Yushchenko led mass protests last November in the "Orange Revolution" after a fraud-plagued presidential election was initially won by the pro-Russia candidate.

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