At least five people were killed in a blast Monday in the rebel-held local administration building in Ukraine's eastern city of Lugansk, officials said.
"Five people were killed. Many people among civilians were wounded," Xinhua cited Konstantin Knyrin, head of the information centre, as telling Interfax-Ukraine news agency.
A fighter jet attack was possibly behind the explosion, sources said.
The sources claimed Ukrainian fighter jets were seen flying over the building prior to the blast.
However, Alexei Dmitrashkovsky, spokesman of the government's military operation in eastern regions, ruled out the possibility of an Ukrainian attack.
"Careless and inept handling of small arms and explosives was the most likely cause of the explosion," he said.
Reports in social media said a huge fire broke out following the blast and many wounded people might still be trapped in the building.
Firefighters and ambulances reached the site.
There has been no official word on the cause of the incident and the exact number of casualties.
Fierce clashes between Ukrainian government troops and armed activists are underway in Lugansk and its neighbouring Donetsk region since early April, after insurgents established "people's republics" in their regions, seeking independence from Kiev.
