KIEV, Ukraine — Russian military forces have been spotted in both major rebel-held cities in eastern Ukraine, an official said Tuesday, prompting Ukraine to declare that it now has to fight the Russian army, not just the separatists.

The statement on the Russians by Col. Andriy Lysenko, a spokesman for Ukraine’s National Security Council, came after the country’s defense minister said Ukraine’s armed forces are expanding their strategy from just fighting separatists to facing the Russian army in a war that could cost “tens of thousands” of lives.

Lysenko told reporters Russian troops had been seen in the cities of Donetsk and Luhansk, as well as other locations throughout the east. The claim could not be confirmed independently. Lysenko also said 15 servicemen had been killed over the previous day.

In Moscow, a Kremlin aide sharply criticized EU Commission President Jose Manuel Barroso for breaching confidentiality when he quoted Russian President Vladimir Putin as saying that Moscow could take over Kiev in two weeks if it wished.

Yuri Ushakov, the Russian leader’s foreign policy adviser, told reporters that Putin’s statement was “taken out of context and carried a completely different meaning.”

Ushakov lashed out at Barroso, saying it was a breach of diplomatic practices and “unworthy of a serious politician” to speak publicly about a private conversation. Barroso had briefed the EU’s 28 leaders hours after the phone conversation at a summit in Brussels — from where the information eventually leaked.

Putin’s comment reportedly came in response to Barroso pointing out Ukrainian and Western reports that Russia had sharply escalated the conflict in eastern Ukraine by sending regular army units into Ukraine. NATO estimates that at least 1,000 Russian soldiers have entered Ukraine, helping turn the tide in the last week in favor of the pro-Russian insurgents. The military alliance also says 20,000 other Russian soldiers have been positioned along the Ukraine-Russian border.