THE HAGUE, NETHERLANDS -- Three more nations on Tuesday joined an international investigation team probing war crimes in Ukraine, and the International Criminal Court prosecutor said he plans to open an office in Kyiv, amid ongoing calls for those responsible for atrocities since Russia's invasion to be brought to justice.

Estonia, Latvia and Slovakia signed an agreement during a two-day coordination meeting in The Hague to join Lithuania, Poland and Ukraine in the Joint Investigation Team that will help coordinate the sharing of evidence of atrocities through European Union judicial cooperation agency Eurojust.

ICC Prosecutor Karim Khan said the teamwork underscores the international community's commitment to the rule of law.

"I think it shows that there is this common front of legality that is absolutely essential, not just for Ukraine ... but for the continuation of peace and security all over the world," he said.

Moscow's invasion of Ukraine has been widely condemned as an illegal act of aggression. Russian forces have been accused of killing civilians in the Kyiv suburb of Bucha and of repeated attacks on civilian infrastructure including hospitals and a theater in the besieged city of Mariupol that was being used as a shelter by hundreds of civilians. An investigation by The Associated Press found evidence that the March 16 bombing killed close to 600 people inside and outside the building.

Since Russia invaded on Feb. 24, the AP and PBS series Frontline have verified 273 potential war crimes.

Ukrainian President Volodymyr Zelenskyy has denounced killings of civilians as "genocide" and "war crimes," while U.S. President Joe Biden has called Russian President Vladimir Putin "a war criminal" who should be brought to trial.

The team that met Monday and Tuesday at Eurojust's headquarters in The Hague was established in late March, a few weeks after the ICC opened an investigation in Ukraine, after dozens of the court's member states threw their weight behind an inquiry. Khan has visited Ukraine, including Bucha, and has a team of investigators -- the largest team of prosecutors ever deployed by the international court -- in the country gathering evidence.

Khan now plans to work toward opening an office in Ukraine "in the next few weeks."

Ukraine's prosecutor general, Iryna Venediktova, said that her office has already opened some 15,000 criminal investigations related to the war and identified over 500 suspects, including Russian ministers, military commanders and propagandists. She said her office was ready to proceed against some 80 of them.

Last week, in the first case of its kind linked to the war, a Ukrainian court sentenced a captured Russian soldier to the maximum penalty of life in prison for killing a civilian. On Tuesday, a court in Ukraine convicted two Russian soldiers of war crimes for the shelling of civilian buildings and sentenced both to 11 1/2 years in prison.

Russia staunchly denies its troops are responsible for atrocities. The Defense Ministry said earlier this month that "not a single civilian has faced any violent action by the Russian military."

Analysts warn that the process of meting out justice will be long and complex as investigators piece together forensic and other evidence and seek to establish who ordered or knew about atrocities and failed to act to prevent or punish them.

The meeting in The Hague isn't the only place accountability is being sought.

Prosecutors in Poland, Germany, Lithuania, Latvia, Estonia, France, Slovakia, Sweden, Norway and Switzerland have opened investigations of their own. And there have been growing calls to set up a special tribunal to try Russia for the crime of aggression in Ukraine. The ICC can't prosecute the crime of aggression because neither Russia nor Ukraine are members of the court.

Khan said the united front of nations investigating crimes since the Russian invasion "hopefully can provide some modicum of accountability for the crimes that we are seeing in Ukraine and that really should no longer be tolerated."