Seventeen apartment buildings were evacuated Saturday in a Russian city near the Ukrainian border after an explosive device was found at the site where a bomb accidentally dropped by a Russian warplane caused a powerful blast this week, authorities said.

The bomb blast late Thursday rocked part of Belgorod, leaving a large crater and three people injured. The Russian Defence Ministry quickly acknowledged that a weapon accidentally released by one of its own Su-34 bombers caused the explosion.

The ministry said an investigation was underway but did not elaborate on the details of the weapon, which military experts said likely was a powerful 500-kilogram (1,100-pound) bomb.

The governor of Belgorod province, Vyacheslav Gladkov, reported Saturday that sappers examining the site of Thursday’s blast found and decided to detonate what he called an “explosive object” that was “in the immediate vicinity of residential buildings.”

The precautionary evacuations ended later in the day, according to Belgorod Mayor Valentin Demidov.

“The bomb was removed from the residential area. Residents are being delivered back to their homes,” Demidov wrote on Telegram.

Russian authorities did not say if the detonated device was dropped by accident on Thursday and if so, if it was a remnant of or separate from the bomb that exploded in the city.

Belgorod, located about 40 kilometres (25 miles) east of the Russia-Ukraine border, has faced regular drone attacks since Russia sent troops into Ukraine last year. Russian authorities have blamed those strikes on the Ukrainian military, which refrained from directly claiming responsibility for the attacks.

Late Saturday, the governor of the Kharkiv region, Oleh Syniehubov, said five missiles fired from the Belgorod area hit the region, including one that struck unspecified “civilian infrastructure” in the capital city Kharkiv.

Moscow’s invasion of Ukraine has sent relations with the West into deep freeze, with frequent expulsions of diplomats on both sides.

On Saturday, the Russian Foreign Ministry said that German authorities had “decided on another mass expulsion of employees of Russian diplomatic missions in Germany.”

A ministry statement said that “as a reaction to the hostile actions of Berlin,” Russia decided to “mirror” the expulsions by Germany and “significantly limit” the maximum number of staff at German diplomatic missions in Russia.

Foreign Ministry spokesperson Maria Zakharova said Russia is expelling more than 20 German diplomats, Russian state media reported, but didn’t give a precise number.

Germany’s Foreign Ministry said it took note of the comments. It said that the German government and Russia had been in contact in recent weeks on “questions regarding the staffing of the respective diplomatic missions” and that a flight on Saturday took place in that context. It didn’t elaborate.

The German air force said earlier that a Russian plane flew to Berlin with diplomatic clearance on Saturday, but didn’t specify who or what was on board. Special clearance is required because the European Union closed its airspace to Russian aircraft shortly after the war in Ukraine started.