Ukraine and Russia have held their fourth round of negotiations for a cease-fire, an adviser to Ukraine's president said Monday, as Moscow's invasion of the East European country enters its third week with civilian casualties growing.

The talks paused later Monday and will continue Tuesday, Reuters reported, quoting Mykhailo Podolyak, adviser to Ukrainian President Volodymyr Zelenskyy. The pause is for "additional work in the working subgroups and clarification of individual definitions. Negotiations continue," he was quoted as saying.

The parties are engaged in "negotiations...on peace, ceasefire, immediate withdrawal of troops & security guarantees," Podolyak said earlier on Twitter.

People are evacuated to an underground shelter in Lviv, western Ukraine, following an air-raid siren on March 13, 2022, amid the Russian invasion of the country. (Kyodo) ==Kyodo

Referring to the latest online discussions between high-ranking officials from the two countries, he tweeted that "communication is being held yet it's hard."

Previous cease-fire talks between Kyiv and Moscow have so far failed to yield significant results, including when Ukrainian Foreign Minister Dmytro Kuleba and his Russian counterpart Sergey Lavrov met in person last week for the first time since Moscow launched its attack on Ukraine on Feb. 24.

The latest cease-fire negotiations come as Moscow's aggression continues to intensify, with Russian forces attacking on Sunday a military base in Yavoriv, western Ukraine, some 20 kilometers from the border with Poland, a North Atlantic Treaty Organization member.

At least 35 people died and more than 100 others were injured when the base was hit by dozens of Russian missiles, Ukrainian officials said.

The Office of the U.N. High Commissioner for Human Rights officially reported as of Sunday that 596 civilian had died in the war, although the actual figure is thought to be higher.

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