ABC News' Alex Marquardt reports from Izium, Ukraine:

Ukraine's military launched an offensive today to quell pro-Russian violence that has swept eastern Ukraine, as Russia's prime minister tweeted an ominous message.

"There is foreboding of a civil war in #Ukraine," Prime Minister Dmitriy Medvedev posted today.

In what Ukraine's acting president has called an "anti-terrorist operation," Ukraine's military - complete with helicopters loaded with special forces units - quickly won back an occupied airfield, its first target.

WATCH: Military prepares for "anti-terrorist operation."

There were reports of wounded pro-Russian protesters, raising fears in Washington that Moscow would use bloodshed as an excuse to invade Ukraine. Russia has repeatedly warned it may intervene to defend Russian speakers in Ukraine.

The US warned of more possible sanctions today against Russia for meddling in Ukraine and appeared cautiously supportive of the operation by Ukrainian security forces in eastern cities.

"We are obviously evaluating requests and looking at ways that we can support the Ukrainian government," said White House press secretary Jay Carney.

"But our focus is on continuing to put pressure on Russia so that it understands that the international community is united when it comes to support for Ukraine's sovereignty and territorial integrity, and that there is a path for Russia to take that would de-escalate the situation and ensure that it doesn't devolve into violence."

Amid the unrest and counter-offensive life largely continued as normal. Most do not support the violence and fear it may spiral.

"I don't want troops to be here," said Kosta Kolsenik in the seized town of Sloviansk. "Really. It's really dangerous for our people."

ABC News' Devin Dwyer contributed to this story.