At least two people were killed and 29 were injured Monday morning when a pair of long-range Russian missiles slammed into buildings in a city in eastern Ukraine, Ukrainian officials said.

The two S-300 Russian missiles hit administrative and office buildings, and private homes in Sloviansk, according to Pavlo Kyrylenko, the regional governor.

Sloviansk is in the Donetsk region of eastern Ukraine, where heavy fighting has been waged since the start of the war.

The missiles struck the city around 10:30 a.m. local time, Kyrylenko said.

He said the town of Druzhkivka in the Donetsk region was also targeted in Monday's missile attacks. Kyrylenko said a Russian missile "almost completely destroyed" an orphanage in Druzhkivka, but there were no immediate reports of casualties.

"Another day that began with terrorism by the Russian Federation," Ukrainian President Volodymyr Zelenskyy said in a statement.

Zelenskyy said Ukraine "will not forgive the torturing of our people."

"All Russian terrorists will be defeated," Zelenskyy said. "Everyone involved in this aggression will be held to account."

ABC's Will Gretsky

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

Ukrainian Defense Minister Oleksii Reznikov said Friday the military is "ready" to launch a counteroffensive against Russian forces.

"It's up to the general staff and the command," Reznikov said during a press briefing in Kyiv. "We will do it as soon as there is God's will, the weather and the commanders' decision."

Ukraine has received Patriot missile defense systems from the United States as well as Germany and the Netherlands. The Ukrainian military has been trained on how to use the systems and "mastered" them within weeks, according to Reznikov.

"The exact number of batteries, I'm sorry, I won't say," he added. "Let the enemy guess."

Meanwhile, Ukrainian Foreign Minister Dmytro Kuleba urged the world not to consider or call the anticipated counteroffensive "a decisive battle." Speaking at a press conference in Odesa on Friday, Kuleba said the decisive battle is the one that will lead to the liberation of all occupied Ukrainian territories.

-ABC News’ Will Gretsky