Russia-Ukraine updates: Putin suspends key US-Russia nuclear treaty in speech denouncing West

Ukrainian forces have launched a major counteroffensive in multiple directions in the southern part of Ukraine, Natalia Humeniuk, a spokesperson for Ukraine's Operational Command, said Monday.

Humeniuk said the situation in the south remains "tense," but controlled.

Ukrainians have been targeting strategic Russian command posts and slowly advancing toward Kherson for weeks. Kherson was first major city in the south to be captured by Russian forces following the invasion.

Russian military issued a statement confirming the offensive and claiming Ukraine sustained heavy losses.

Meanwhile, at least 12 missiles have struck Mykolaiv, which remains under Ukraine's control in the south. Two people were killed and 24 were wounded, according to the governor of Mykolaiv Oblast.

-ABC News' Max Uzol and Natalia Shumskaia

Oleksiy Kovalyov, a Ukrainian official who was accused of treason for openly collaborating with Russia, was shot and killed in his home on Sunday in Hola Prystan, Kherson Oblast, according to preliminary information from the Investigative Committee of Russia (SKR). An unidentified woman was also killed, SKR said.

Kovalyov was a Ukrainian lawmaker from President Volodymyr Zelenskyy's party who was accused of treason; criminal proceedings were initiated by Ukraine's State Bureau of Investigations in June. He is one of the highest-ranking Ukrainian defectors who fled to Kherson after the invasion and openly collaborated with Russia. He was appointed by the Russians as the deputy head of the Kherson Military-Civil Administration.

The head of the United Nations' nuclear watchdog announced Monday that the agency's long-awaited expert mission to the Zaporizhzhia power plant in southeastern Ukraine "is now on its way."

"The day has come," Rafael Grossi, director general of the International Atomic Energy Agency, said in a post on Twitter.

Grossi, who is leading the IAEA's "Support and Assistance Mission to Zaporizhzhia," has long sought access to the nuclear power plant, which is the largest in Europe. Russia and Ukraine have traded accusations of shelling at or near the site in recent weeks, fueling fears that the fighting could cause a nuclear disaster.

"We must protect the safety and security of #Ukraine's and Europe's biggest nuclear facility," Grossi tweeted, alongside a photo of himself with 13 other experts. "Proud to lead this mission which will be in #ZNPP later this week."

Shortly after invading neighboring Ukraine on Feb. 24, Russian troops stormed the Zaporizhzhia nuclear power plant near the town of Enerhodar, on the banks of the Dnipro River in the country's southeast. The Ukrainian workers have been left in place to keep the plant operating, as it supplies electricity across the war-torn nation.

Kremlin spokesperson Dmitry Peskov said the IAEA team will travel to the plant via Ukrainian-controlled territory, state-run TASS reported.

The area around the nuclear plant is controlled by Russian forces. Peskov said once the IAEA team enters Russian-controlled territory, all necessary security will be provided.