“The world is beginning to recognize that (Russia) is a terrorist state. And terror knows no borders,” Defense Minister Oleksiy Reznikov said. “They use terrorist means to cause terror.”

Reznikov added that he had spoken with his Spanish counterpart Margarita Robles about the recent explosions.

On Dec. 1, Spain said it was increasing security measures following the incidents, CNN reported.

Earlier on the same day, yet another letter bomb was found at the U.S. embassy in Madrid, according to the Spanish newspaper El Pais.

The letter was left at the security post of the embassy, the report reads.

Previously, similar explosives were sent to Spanish Prime Minister Pedro Sanchez and Defense Minister Margarita Robles.

Other bombs were sent to the Ukrainian embassy in Madrid; the Torrejon de Ardoz air base in Madrid, which provides intelligence information to Ukraine's Armed Forces, and the headquarters of Spanish weapons manufacturer Instalaza, which produces rocket launchers supplied to Ukraine.

One of the Ukrainian embassy's employees received minor injuries while inspecting the bomb, according to Foreign Ministry spokesperson Oleh Nikolenko.

The Spanish High Court has already opened a probe into the attack. According to the Spanish authorities, all of the envelopes contained similar, homemade substances.

According to Spanish media, officials are investigating whether the incidents are linked to the Russian invasion of Ukraine.