The appeal by the military alliance on Friday is in spite of the former Soviet nation being severely hit by the economic crisis.
"Allies are aware that the financial and economic crisis that has also hit Ukraine very hard is having an effect on Ukrainian reforms," Jaap de Hoop Scheffer, NATO's secretary general, said after talks with defence ministers in Krakow, Poland.
"Ministers encouraged Ukraine to keep up the effort including with the necessary resources, difficult as that may be, because an inefficient and antiquated defence and security sector will cost Ukraine much more in the long run," he said.
The global economic downturn has seen Ukraine's industrial output sink by a third and its currency plummet in value, while political turmoil has jeopardised part of a $16.4bn loan from the International Monetary Fund (IMF).
The alliance has said that Ukraine and Georgia can one day join the 26-member bloc, amid opposition from Russia over NATO's expansion into former Soviet nations.
Scheffer said NATO officials were looking at "ways in which the alliance can continue to support its preparations for NATO membership" for Ukraine.
Robert Gates, the US defence secretary, said Ukraine still had a long way to go before it could join the alliance, but that the Obama administration had no "significant quarrel" with its attempt to become a member.
"There is a long path in front, and frankly there has to be greater unanimity of views within the Ukrainian government itself about the next steps," he said.
Bogdan Klich, Poland's defence minister and host of the two-day meeting, said his country would continue to lobby hard for the two countries to join NATO.
But Ulrich Schlie, the German defence ministry's director of planning, expressed his concerns over NATO's continued expansion.
"The larger the alliance gets the more difficult it is to maintain cohesion," he said.
Moscow, which already has strained ties with NATO, has been angered by the alliance's open-door policy in regard to Georgia and Ukraine, which both lie on Russia's southern border.
Source: Al Jazeera