Mikhail Koval made the remarks as Russian President Vladimir Putin said accusations of his country's involvement in Ukraine were "unfounded".
Koval said in a televised address to the nation on Monday that the armed forces would act without imposing a martial law in the framework of the anti-terrorist operation.
"The reasons for using armed forces exists," he said.
The announcement came a day after Ukraine's acting President Oleksandr Turchynov warned pro-Russian activists in the country's eastern cities on Sunday that a full-scale security operation would include the army would be unleashed unless they put down arms until 0600GMT on Monday.
Armed rebels defied the deadline, storming a police station in Horlivka and continuing to occupy government offices in at least nine other eastern towns.
On Monday, Turchynov suggested in a telephone conversation with Ban Ki-moon, UN secretary-general, that UN peacekeepers could get involved in stabilising the country's east.
Meanwhile, Russia's envoy to the Organisation for Security and Cooperation in Europe (OSCE) said on Monday that any use of armed force against pro-Russian demonstrators could spark a civil war in Ukraine.
"There are units being organised of paramilitary people who will be given weapons and who will be under command of officers. It will be, as we hears, nearly 12,000 of these people. This is dangerous," Andrey Kelin said.
"In Moscow, we strongly believe in might lead to a civil war. We are very worried."
Blaming the Kremlin
The EU discussed fresh sancrions against Russia on Monday as foreign ministers blamed Russia for fomenting trouble in restive eastern Ukraine.
Events of the last 48 hours in eastern cities of Ukraine are "clearly a further escalation of the crisis", William Hague, British foreign secretary, said on arriving for the talks with his 27 counterparts.
There could be no "doubt that this has been planned and brought about by Russia," he said, adding that Russian denials of involvement "do not have a shred of credibility".
Later on Monday, Russia's President Vladimir Putin told US counterpart Barack Obama that charges of Russia meddling in eastern Ukraine were "unfounded", the Kremlin said, referring to a telephone conversation between the two leaders.
