Around 12,000 people were evacuated after a fire and explosions at a rate of two to three a second hit a Ukrainian Defence Ministry ammunition depot early on Tuesday morning, officials said.
No casualties were reported.
Ukraine's state security service said it was investigating possible sabotage, and the defence ministry's spokesman said the fact that explosions were set off in different parts of the depot pointed to sabotage.
The depot is located in the Chernihiv region, 176 km (109 miles) east of the capital, Kiev. A woman who lived 50 km away told the TV channel 112 she could hear the explosions.
"There are no victims, wounded, injured or killed among military personnel, personnel of the Armed Forces of Ukraine and the local population," a Defence Ministry statement said. "As of 7 a.m. (0400 GMT), the intensity of explosions is two to three explosions per second."
The airspace in a 30 km radius was closed and road and rail transport suspended. The emergency services reported gas and electricity supplies to the area have been disrupted.
Hundreds of people and equipment were deployed to the site, a statement by the emergency services said, joined by Prime Minister Volodymyr Groysman and the head of Ukraine's armed forces Viktor Muzhenko. The president has called for a report.
Several large fires have hit ammunition and weapons depots in recent years, an additional drain on Ukraine's military. Fighting between Ukrainian troops and Moscow-backed separatist rebels has killed more than 10,000 people since 2014.
Last year, massive explosions at a military depot in the Vynnytsya region, 270 km west of Kiev, forced the authorities to evacuate 24,000 people.
Following that, a parliamentary defence committee inspected other depots. It warned that there were significant shortcomings in how the depot in the Chernihiv region was managed, according to a lawmaker who was on the committee. A "number of shortcomings, including significant ones, were identified," Dmitry Tymchuk wrote on Facebook,.
"As a result of this trip, I sent an address to the heads of the Ministry of Defence and the General Staff of the Armed Forces, which listed the shortcomings identified with a request to intervene in the situation and solve existing problems."
The defence ministry did not immediately respond to requests for comment from Reuters.