It left a 20-meter (66-foot)-wide crater in the middle of a tree-lined avenue flanked by apartment blocks, 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-story 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.

Some alleged that the bomb that was accidentally dropped on Belgorod could be one of a batch of modified munitions equipped with wings and a GPS-guided targeting system that allows them to glide to targets dozens of kilometres (miles) away. The Russian air force has started using such gliding bombs only recently, and some experts say that they could be prone to glitches.

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. IgMp