Belarus has no plans to join Russia's military operation in Ukraine, Belarusian President Alexander Lukashenko said Tuesday, but noted that the country's air defense systems have been put on alert to "prevent Russian forces from being hit in the back."

Lukashenko denied Kyiv's allegations that Russian troops were attacking Ukraine from Belarus' territory, Belarusian state news agency Belta reported.

He also commented on Monday's meeting between Ukrainian and Russian delegations on the Belarusian border and praised the outcome of the peace talks, despite no progress having been made, leading to an expected second round of negotiations.

"The negotiators are just great," Lukashenko said, according to Belta. "They did more than was possible. They brilliantly did their job in five hours at the first stage," he said.

Despite Belarusian claims that it did not participate in the military assault in Ukraine, the Ukrainian military's General Staff wrote on Facebook it believed troops from Belarus were also moving towards the country.

"Several units from the Belarusian armed forces in the most combat-ready formations have started to move towards the Ukrainian border in the direction of Volhynia," it wrote.

The southern Ukrainian port of Mariupol was back under Ukrainian control, the country's state information service said in the early hours of Tuesday.

Volhynia is in Ukraine's northwest, close to the border with Belarus.

The Ukrainian state information service also said an attack on the southern Ukrainian city of Kherson had begun.

Ukrainian President Volodymyr Zelenskyy said that Russia has carried out 56 rocket attacks on Ukraine since the invasion began five days ago, and fired a total of 113 cruise missiles. Russia, on the other hand, said Ukrainian "nationalists" were shelling the cities.

Both the Ukrainian and Russian armed forces reportedly suffered losses as Moscow attacked the Sumy region of northeastern Ukraine.