Nato Secretary General Jaap de Hoop Scheffer is on a one-day visit to Ukraine for talks on the country's ambition to join the alliance.
Mr Scheffer pledged support for Ukraine's reforms
Mr Scheffer will discuss the reforms Ukraine is implementing to meet the accession criteria for Nato as well as the European Union.
He is due to meet President Viktor Yushchenko and other top officials.
Analysts say Ukraine will need at least five or six years before it can be considered for membership.
"We and the Ukrainian authorities have to concentrate our attention above all on reaching the main goal - carrying out necessary reforms," Mr Scheffer told the Ukrainian weekly Zerkalo Nedeli ahead of his trip.
Earlier this year, Nato drew up a three-page document on "short-term actions" required from Ukraine.
Mr Scheffer said the move was a show of support for the country's drive towards integration with Europe, adding that Nato would support and advise it "along the route".
But he underlined that the success of the process would ultimately depend on Ukraine's efforts.
According to recent opinion polls, Mr Yushchenko's bid to join the alliance is opposed by 55.7% of Ukrainians.
Russia, too, opposes the move, which would extend Nato's borders with Russia beyond the Baltic republics.
Ukraine currently supplies Russia with helicopters, gas turbines for its vessels and parts of anti-aircraft systems.
Mr Yushchenko, a pro-western liberal, was swept into power last year in the so-called "Orange Revolution", defeating a pro-Russian rival.