Russian forces have closed in further on Kharkiv, gaining about 20 kilometers, about 12 miles, since Tuesday, a senior U.S. defense official told reporters at a press briefing Wednesday.

"It's still heavy, heavy fighting there," the official said on Day 14 of the Russian invasion. However, the U.S. does not believe Russia has "taken the city by any means," the official added.

The U.S. has not seen any significant movement toward Kyiv or Chernihiv since Tuesday, and there is still no sign of an any immanent amphibious assault on Odessa, the official said.

In southeast Ukraine, Mariupol remains isolated. Ukrainians are putting up stiff resistance, while Russians continue to bombard the city, the official said.

Russia has about 90% of its available combat power still intact, the official said. It has now launched more than 710 missiles against Ukraine -- about half being launched from within Ukraine and half being launched from within Russia, the official said.

-ABC News' Matt Seyler

Russia is retaining a temporary ceasefire in humanitarian corridors in Ukraine, head of the Russian National Defense Control Center Col. Gen. Mikhail Mizintsev alleged on Wednesday.

Ten of the corridors were proposed by Moscow, while three were proposed by Kyiv, Mizintsev told reporters at a press briefing.

"Today, at exactly 10 a.m. local time, in accordance with agreements reached on 10 humanitarian corridors initiated by the Russian Federation and three more at the request of the Ukrainian side, the Russian Armed Forces introduced a temporary ceasefire, which is being strictly observed until now," Mizintsev said

No practical measures for the preparation of humanitarian convoys have been carried out so far, and no one has ever arrived at the humanitarian corridors that have been opened, Mizintsev said. The formation of organized humanitarian convoys has also not been recorded, he claimed.

A devastating Russian airstrike has destroyed a maternity and children’s hospital in the besieged Ukrainian city of Mariupol, according to Ukrainian officials.

Ukraine’s president Volodymyr Zelenskyy and local officials have posted videos on social media of what appears to be the devastated hospital, partly caved in and with walls blown out, the grounds littered with burnt-out vehicles.

ABC News is attempting to independently verify the attack.

Ukrainian authorities have said there are casualties and Zelenskyy wrote that there are children under the rubble.

Ukrainian officials have accused Russia of deliberately targeting the hospital.

Pavlo Kirilenko, the head of the Donetsk regional military administration, wrote on Facebook that a Russian aircraft targeted the hospital. Other officials have suggested there were multiple strikes.

Zelenskyy has called it an “atrocity” and called on the international community again to impose a "no-fly" zone.

"Direct strike of Russian troops at the maternity hospital. People, children are under the wreckage. Atrocity! How much longer will the world be an accomplice ignoring terror? Close the sky right now! Stop the killings! You have power but you seem to be losing humanity,” Zelenskyy tweeted.

-ABC News' Patrick Reevell

A Russian Defense Ministry spokesman claimed Wednesday that Russia's Aerospace Forces have destroyed more than half of the Ukrainian Armed Forces aircraft.

Russian Defense Ministry spokesman Igor Konashenkov said Russian forces have 89 Ukrainian combat planes and 57 helicopters out of an estimated total of 250 combat aircraft once in possession of the Ukrainian Armed Forces and "in good working order" prior to Russia's special military operation in Ukraine.

"Ninety percent of Ukrainian military airfields, which accommodate the core combat aviation force, have been rendered inoperable," Konashenkov said.

He said that as of Wednesday, Russia has observed "only isolated attempts" of the Ukrainian Air Force's combat aircraft to take off.

Neither U.S. Defense officials nor Ukrainian officials has confirmed Konashenkov's report.

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