MOSCOW — President Vladimir V. Putin of Russia has told a European official that he could “take Kiev in two weeks” if he wanted to, adding a new dimension to the tensions building in Ukraine as Russian forces become more involved in the fighting there.

As NATO leaders gathered in Wales for a summit meeting, Mr. Putin’s remarks and the increasing presence of Russian military units in Ukraine presented NATO with a stark new challenge about how to respond to Moscow’s apparent willingness to exert military force to achieve its foreign policy goals.

The Kremlin did not deny the remark, which was published in the Italian newspaper La Repubblica on Monday, but on Tuesday it denounced the European official, José Manuel Barroso, for leaking details of what Mr. Putin understood to be a private telephone call on Friday.

“Whether these words were said or not, in my viewpoint, the quote given is taken out of context, and it had an absolutely different meaning,” said Yuri Ushakov, a Kremlin aide, according to the Interfax news service.