The Ukrainian government has reluctantly agreed to a series of talks aimed at ending the crisis by giving eastern regions of Ukraine more independence.
Although Russia strongly backs the European-brokered peace plan, Ukrainian officials and the United States remain skeptical.
The Washington Post reports that Kiev refused to invite the separatists to take part in the negotiations, saying it won't hold talks with those who have "blood on their hands."
"The government will act against those who are terrorizing the region with arms in hand, in line with the law, by continuing an anti-terrorist operation against them," acting Ukrainian President Olexandr Turchynov said.
Meanwhile, six Ukrainian soldiers were killed by pro-Russian rebels during an ambush Tuesday in the eastern part of the country.
Heavily armed rebels fired rocket-propelled grenades and automatic weapons at the convoy of Ukrainian paratroopers in Donetsk.
"Our soldiers were attacked in an ambush. Terrorists attacked our land troops with grenades. The attackers were more than 30 people and set an ambush near the river," Ukrainian Defense Ministry said in a statement on its website.
"After a long shootout, six soldiers of The Ukrainian Armed Services were killed," the ministry said.
The attack was the latest in a string of deadly clashes between pro-Russian separatists and Ukraine's interim government.
Dozens have been killed on both sides over the past several weeks.