MOSCOW -- Russia's military acknowledged that a bomb accidentally dropped by one of its warplanes caused a powerful blast in a Russian city not far from Ukraine's border, injuring two and scaring local residents.

Belgorod, a city of 340,000 located about 40 kilometres (25 miles) east of the Russia-Ukraine border, has faced regular drone attacks during Russia's current military operation in Ukraine. Russian authorities blamed the earlier strikes on the Ukrainian military, which refrained from directly claiming responsibility for the attacks.

The explosion late Thursday was far more powerful than anything Belgorod residents had experienced before. Witnesses reported a low hissing sound followed by a blast that made nearby apartment buildings tremble and shattered their windows.

It left a 20-metre (66-foot) -wide crater in the middle of a tree-lined boulevard flanked by apartments, damaged several cars and threw one vehicle onto a store roof. Two people were injured, and a third person was later hospitalized with hypertension, authorities said.

Immediately after the explosion, Russian commentators and military bloggers were abuzz with theories about what weapon Ukraine had used for the attack. Many of them called for strong retribution.

But about an hour later, the Russian Defense Ministry acknowledged that a weapon accidentally released by one of its own Su-34 bombers caused the blast. The ministry did not provide any further details, but military experts said the weapon likely was a powerful 500-kilogram (1,100-pound) bomb.

Military experts charged that the weapon appeared to have been set to explode with a small delay after impact that would allow it to hit underground facilities.

Belgorod Gov. Vyacheslav Gladkov said local authorities decided to temporarily resettle residents of a nine-storey apartment building while it was inspected to make sure it hadn't suffered structural damage that rendered it unsafe to live in.

Russian commentators questioned why the warplane flew over Belgorod and urged the military to avoid such risky overflights in the future.

In October, a Russian warplane crashed next to a residential building in the port city of Yeysk on the Sea of Azov, killing 15 people. Yeysk hosts a big Russian air base with warplanes that fly missions over Ukraine.

Military experts have noted that as the number of Russian military flights have increased sharply during the fighting, so have crashes and misfires.

In another deadly incident in the Belgorod region, two volunteer soldiers fired at Russian troops at a military firing range, killing 11 and wounding 15 others before being shot dead.