One week into the conflict, half a million children have already fled Ukraine to neighboring countries with the number continuing to grow, the United Nations International Children's Emergency Fund said on Thursday.

"Millions of children could be caught up in the violence as the fighting intensifies in and around the country’s major urban centers," UNICEF said in a statement.

Since Feb. 24, 17 children have been killed and 30 others have been injured, according to reports the UN has been able to verify. UNICEF said the true number of casualties is "likely far higher."

“The use of explosive weapons in cities could quickly turn this crisis into a catastrophe for Ukraine’s children,” said Afshan Khan, UNICEF’s Regional Director for Europe and Central Asia.

“There are no armed operations of this scale that do not result in children being harmed. The consequences will be tragic,” Khan said.

The conflict has sparked massive population displacement that could soon constitute one of Europe’s largest refugee crises since World War II, UNICEF warned.

UNICEF said hundreds of homes have been damaged or destroyed and there are reports of schools, orphanages and health centers sustaining heavy damage.

"Humanitarian needs across the country are multiplying by the hour," UNICEF said.

The country is running low on critical medical supplies and has had to halt urgent efforts to curb a polio outbreak, UNICEF said.

- ABC News' Christine Theodorou

When asked by reporters how long Ukraine can hold out against the Russian advance, Ukraine President Volodymr Zelenskyy said "I don't know."

In a press conference on Thursday reporters asked the president if he would ever leave Kyiv, Zelenskyy said "no!"

Zelenskyy said he's ready to meet and talk directly with Russia President Vladimir Putin and stressed once again the need for a 'no-fly' zone immediately over the entire country, saying it would act as a deterrent.

He paid tribute to the valor of Ukrainian forces and appealed to Russian mothers not to send their sons to die on foreign soil.

Zelenskyy also thanked President Joe Biden for the continuing support, but criticized the West for being late to introduce sanctions that he believes could’ve prevented the Russian invasion in the first place.

-ABC News' Ian Pannell

Russia's Defense Ministry claimed the forces it backs in Luhansk and Donetsk, the two Ukrainian regions it recognized as independent states, are advancing and gaining territory.

Russian Defense Ministry Spokesman Igor Konashenkov claimed, in a press briefing on Thursday, that Donetsk forces conducted an offensive advancing eight kilometers and taking control of the localities of Novokrasnovka and Krasnovka.

Russia claims its armed forces and Donetsk units also took control of the localities of Blahoveshchenka, Synya Hora, Petrivske, Sladkaya Balka, and others, according to Konashenkov.

Konashenkov also claimed that Luhansk forces backed by Russia took control of the Yarovaya-Krasnyi Lyman-Prilovye road.

-ABC News' Tanya Stukalova

Russia's Foreign Ministry claimed U.S. military intelligence launched a massive campaign to recruit contractors from private military companies to send to Ukraine.

"They recruit primarily those employed by Academi, CUBIC and DynCorp [International]," Russian Defense Ministry spokesperson Igor Konashenkov said in a press briefing on Thursday.

Foreign mercenaries who have arrived in Ukraine are performing acts of sabotage and attacking Russian convoys of motor vehicles and aviation, Konashenkov said.

"Foreign mercenaries who earlier arrived in Ukraine are carrying out acts of sabotage and raids on Russian convoys of [military] vehicles and materiel supplies, and on the aviation supporting them," Konashenkov said.

Konashenkov claimed Britain, Denmark, Latvia, Poland and Croatia have legally permitted their citizens to take part in Ukraine hostilities and that the command of the French Foreign Legion plans to send its ethnic Ukrainian troops to aid the Kyiv regime.

Mercenaries from other countries in Ukraine will not receive prisoner-of-war status and will be subject to criminal liability, Konashenkov said.

"I want to officially emphasize that all mercenaries sent by the West to help the Kyiv nationalist regime are not combatants under international humanitarian law. They are not entitled to POW status," Konashenkov said.

Russia said it hopes its current talks with Ukraine on Belarusian soil will soon lead to a peaceful settlement, Russian Foreign Ministry spokesperson Maria Zakharova said.

"We hope the [talks] will lead to a swift conclusion of this situation, the restoration of peace in Donbas, and the return of all peoples of Ukraine to a peaceful and equitable life," she told a briefing in Moscow on Thursday.

- ABC News' Tanya Stukalova

U.S. Secretary of State Antony Blinken crossed the border into Ukraine Saturday after meeting with Ukrainian Foreign Minister Dmytro Kuleba at the Korczowa Border Crossing Point.

"The entire world stand with Ukraine," Blinken said, and Kuleba added that he hopes Ukrainians will see this as a "clear manifestation that we have friends who literally stand by us."

Blinken said the sanctions imposed so far against Russia are "producing very, very concrete results" and that the pressure will "grow" until this "war of choice is brought to an end."

When asked directly about helping Kuleba with more firepower, Blinken said, "the support for Ukraine not only has been unprecedented, not only is going to continue, it's going to increase."

Kuleba said Ukraine "appreciates" sanctions that have been announced in the last week, but that more economic and political pressure and "necessary weapons," would "save many lives in Ukraine... many sufferings will be avoided."

Kuleba also said that they are “satisfied" with already arranged supplies of anti-tank weapons and ammunition, but that “it’s no secret” they still want fighter jets and air defense systems.

He also thanked the U.S. for sending stingers, and said they were used on Saturday.

"Just today we shot down three Russian attack aircrafts, which were bombing our cities with the use of stingers but we need -- so to say -- big air defense systems to ensure the safety of our skies. If we lose the skies, there will be much, much more blood on the ground,"Kuleba said.

In response to questions about NATO’s refusal to impose a no-fly zone over Ukraine, Kuleba said it’s a "sign of weakness," but stayed optimistic that they will change their minds.

"The time will come," he said.

-ABC News' Conor Finnegan and Justin Gomez