(Sasha Mordovets/Getty Images)

MOSCOW - Russian President Vladimir Putin offered a vague but ominous warning to Ukraine's government, saying there may be unspecified "consequences" for its use of force against pro-Russian separatists in the east.

"If the Kiev regime has started to use the army against the population inside the country, it, beyond any doubt, is a very serious crime," he said, according to Russia's Interfax news agency.

"It will certainly have consequences for the people who make such decisions," he added. "This also concerns our intergovernmental relations."

His comments renewed fears that Russian troops might cross the border under the pretext of protecting Russian speakers there.

As if on cue, shortly after Putin spoke, his defense minister announced plans for more military drills along the border in Ukraine in response to the unrest.

"We have to react to such a development," Defense Minister Sergei Shoigu said, according to Interfax.

The United States and its allies accuse Russia of fomenting unrest in Ukraine and President Obama said today that the White House has additional sanctions on Russia "teed up" if it does not stop its meddling.

Putin was defiant in the face of additional Western sanctions, saying the measures imposed so far have not had a "critical" impact on Russia's economy.

"No sanctions are effective in the modern world and they never have a lasting effect," Putin said. "Rather, this is all political in nature."

The Russian leader added that Ukraine's military actions in the east justify Russia's decision to annex Crimea.

"Otherwise, they would have witnessed the same events as eastern Ukraine and surely even worse. So, this is another proof that we have acted correctly and on time," Putin said. "If Russia had not rendered real support to people in Crimea, it would have been impossible to organize a civilized process of the expression of people's will there."