Firefighters could not contain the blaze and it spread to the arsenal which contained about 100 tons of ammunition on a 1,235-acre site, according to the statement.
About 1,400 people had been evacuated and 6,000 would be evacuated from the area around the facility, officials said.
Scared residents
Lidya, a local resident, said: "They told us on radio and TV we should take documents and personal belongings and leave our houses to wait near bus stops."
"People waited and waited but no transportation was provided. So people started to run, all, including disabled people and old people. I was standing here and saw everything - it was scary, I've never seen anything like that."
Nikolai Tityursky, a spokesman for the Ukrainian armed forces, said: "We have conducted a helicopter reconnaissance flight to detect the locations where the burning process is still under way, where there are still rounds of ammunition that can detonate."
"After the explosions finish, we'll send five tanks to help put out the blaze."
Russia's RIA-Novosti news agency quoted Yulia Tymoshenko, the prime minister, as saying there were no casualties, and no one was seriously injured.
Ukrainian defence officials have warned that dozens of large ammunition depots inherited from the former Soviet Union are poorly maintained and represent a serious public hazard.
A fire and explosions at a munitions depot in southern Ukraine killed five people in 2004.