Three people were injured in an explosion in the Kireevsky district of the Tula region on Sunday, Yekaterina Makarova, press secretary of the region's Ministry of Health, told Interfax.

Russian authorities and law enforcement agencies said a Ukrainian drone with ammunition caused the explosion in the town far from the two countries' border.

Kireevsk is about 180 miles from the border with Ukraine and 110 miles south of Moscow.

The Russian state-run news agency Tass reported authorities identified the drone as a Ukrainian Tu-141. The Latvia-based Russian news outlet Meduza reported that the blast left a crater about 50 feet in diameter and 16 feet deep.

-ABC News' Anastasia Bagaeva

Russia called Slovakia's transfer of MiG-29 fighter jets to Ukraine an unfriendly step and a step aimed at destroying bilateral relations.

"We are talking about another gross violation by the Slovak side of its international obligations to re-export Russian-made weapons and military equipment," Russia's Federal Service for Military-Technical Cooperation said in a statement.

"We regard these actions of Slovakia as an unfriendly act against the Russian Federation, aimed at destroying bilateral relations," the FSMTC said.

-ABC News' Will Gretsky

Ukrainian troops, on the defensive for four months, will launch a long-awaited counterassault "very soon" now that Russia's huge winter offensive is losing steam without taking Bakhmut, Ukraine's top ground forces commander Colonel-General Oleksandr Syrskyi said Thursday.

"The aggressor does not give up hope of taking Bakhmut at any cost, despite the losses in manpower and equipment," Syrskyi said.

Adding, "Without sparing anything, they lose considerable strength and exhale. Very soon we will take advantage of this opportunity, as we once did near Kyiv, Kharkiv, Balaklia and Kupyansk."

-ABC News' Will Gretsky

Slovakia has handed over four MiG-29 fighter jets to the Ukrainian Armed Forces, according to Slovakian Defense Minister Jaro Nad.

The remaining aircrafts promised to Ukraine will be handed over in the following weeks, Nad said.

In response to the news, Russia accused NATO and the EU of continuing to escalate the conflict in Ukraine and seeking to prolong it.

"The Russian Federation considers the transfer of four fighter jets by Slovakia to Ukraine a destructive step that runs counter to the EU's rhetoric about seeking peaceful solutions," Deputy Foreign Minister Alexander Grushko said in a statement.

Adding, "The Russian Federation will measure its reaction with the specific military activities of NATO on the territory of Finland."

-ABC News' Will Gretsky

Ukrainian President Volodymyr Zelenskyy said the Ukrainian counteroffensive will start before Ukraine receives F-16 fighter jets sought from Western countries.

"Frankly speaking, it would help us a lot. But we also understand that we can't drag it [the counteroffensive] out, which is why we'll start before we receive F-16 [aircrafts] or other models," Zelenskyy said at a news conference for Scandinavian media held in Kyiv on Saturday, according to a script provided by Reuters.

He added, "But to calm Russia down with the fact that we'd still need a couple of months to train on the aircrafts and only then we'd start; No, this won't happen. We'll start and go forward, while at the same time, simultaneously, I think this is very important [to receive western fighter aircrafts.]"

He said Ukraine is "capable of putting an end to this war."

Also on Saturday, Ukraine's Foreign Minister Dmytro Kuleba said Ukraine received a signal from some countries about the readiness to start training Ukrainian pilots on F-16 fighter jets.

According to Kuleba, the F-16 fighter jet is the ideal aircraft due to its technical characteristics, although Kyiv does not overlook other aircraft, either. The minister added that the decisive word on issuing F-16s will be with the United States, because these are American fighters.

-ABC News' Edward Szekeres and Max Uzol