Russia has accidentally bombed its own city, injuring two people.
Residents of Belgorod, a city of 340,000 near Ukraine, first thought it was a Ukrainian attack but the Russian military quickly acknowledged one of its warplanes accidentally released the weapon.
Belgorod has faced regular drone attacks that Russian authorities have blamed on Ukraine, but the explosion late on Thursday was far more powerful than anything its residents had heard before.
Witnesses reported a low hissing sound followed by a blast that made nearby apartment buildings tremble and threw a car on a shop roof.
The bomb left a 66ft-wide crater in the middle of a tree-lined boulevard flanked by apartment buildings, shattering their windows, damaging several cars and injuring two residents.
A third person was later admitted to hospital with hypertension.
Immediately after the blast, Russian commentators and military bloggers were abuzz with theories about what weapon Ukraine had used for the attack.
Many called for a powerful retribution.
But about an hour later, the Russian Defence Ministry acknowledged that the explosion was caused by a weapon accidentally dropped by one of its own Su-34 bombers.
It did not offer any further details but military experts said the weapon was likely a powerful 500kg bomb.
In Thursday’s explosion, the weapon was apparently set to explode with a small delay after impact in order to hit underground facilities.
Belgorod governor Vyacheslav Gladkov said local authorities decided to temporarily resettle residents of a nine-storey apartment building near the blast while it was inspected to make sure it had not 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.
The explosion in Belgorod followed the crash of a Russian warplane next to a residential building in the port city of Yeysk on the Sea of Azov that killed 15 people.
Yeysk hosts a big Russian air base with warplanes flying 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 accidents.
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.