The head of the United Nations Children's Fund (UNICEF) strongly condemned a missile attack apparently launched by Russia Wednesday on a maternity hospital in Ukraine.

"I am horrified by the reported attack today on a maternity hospital in Mariupol, Ukraine -- an attack which reportedly left young children and women in labor buried beneath the rubble of destroyed buildings," UNICEF executive director Catherine Russell said in a statement. "We do not yet know the number of casualties but fear the worst."

A devastating airstrike destroyed a maternity and children’s hospital in the besieged southeastern Ukrainian city, causing multiple casualties. Ukraine’s president Volodymyr Zelenskyy and local officials posted videos on social media of the destroyed hospital.

Zelenskyy called the attack an "atrocity."

Russell said that if confirmed, the attack "underscores the horrific toll this war is exacting on Ukraine’s children and families."

"In less than two weeks, at least 37 children have been killed and 50 injured, while more than 1 million children have fled Ukraine to neighboring countries," Russell said.

Russell added, "Attacks against civilians and civilian infrastructure --including hospitals, water and sanitation systems and schools -- are unconscionable and must stop immediately. UNICEF renews its call for an immediate cease-fire and urges all parties to respect their obligations under international humanitarian law to protect children from harm, and to ensure that humanitarian actors can safely and quickly reach children in need. The children of Ukraine desperately need peace."

Russia has confirmed the use of thermobaric rockets, or vacuum bombs, the U.K. Ministry of Defense tweeted Wednesday.

The weapon uses a fuel container and two separate explosive charges to ignite a blast of extreme pressure and heat, creating a partial vacuum in an enclosed space.

The impact from the bomb is "devastating," according to the ministry.

-ABC News' Guy Davies

Papa John's and Heineken are the latest brands to cease business in Russia in response to the invasion of Ukraine.

The American pizza chain and the Dutch beer company join internationally known brands such as McDonald's, The Coca-Cola Company, PepsiCo and Starbucks in suspending operations in Russia.

While Heineken has stopped the production and sale of beer in Russia, Papa John’s has suspended all corporate operations there as well.

"It has ceased all operational, marketing and business support to, and engagement with, the Russian market, where all restaurants are owned by independent franchisees, and a master franchisee who controls operations and provides all supplies and ingredients for the restaurants through a supply chain that it owns and operates," a statement released Wednesday by Papa John’s read.

-ABC News' Victor Ordonez

Russian forces severed the electricity to the Chernobyl nuclear power plant but there is "no immediate risk" of a radiation leak, International Atomic Energy Agency (IAEA) Director-General Rafael Mariano Grossi told ABC News in an interview Wednesday at the agency’s headquarters in Vienna.

There is active fuel at the site of the original reactors that melted down in 1986 but, for the time being, at least, there is enough capacity to cool spent nuclear fuel.

"There is no immediate risk in the dimensions that were imagined and there is work in progress to restore the electrical capacity," Mariano Grossi said. Still, he conceded, "It's a situation that is very fragile."

The lack of reliable electricity also impedes monitoring abilities, leaving the IAEA -- the nuclear watchdog of the United Nations -- occasionally blind to any spontaneous increase in radiation.

"We do have communications and then we lose. Then we recover it. It's not good in terms of following the safety, the security," Mariano Grossi said. "I'm concerned. I’m worried."

There are no IAEA inspectors on the ground while the fighting rages. The director-general said he would not send them in unless he can go first.

"I will not put my staff in harm’s way before me going first," Mariano Grossi said.

Russia is in control of Chernobyl and a second nuclear power station in Zaporizhia. For a time, neither plant had a way to exchange workers or upgrade staffing.

In Zaporizhia, Mariano Grossi said, a shift change is now happening. In Chernobyl, workers are not allowed off-site.

"We all need a break and especially people who are manning extremely sophisticated equipment. The stress is very high," he said.

Mariano Grossi is traveling Thursday to Turkey, where the Russian and Ukrainian foreign ministers will meet face-to-face for the first time since the two-week-old invasion began. He said he will ask each side to commit to "fundamentals," including respecting the physical security of each of Ukraine’s four nuclear power stations, plus Chernobyl.

"We haven't seen something as critical and worrying as a fire breaking out in a building adjacent to a nuclear reactor," Mariano Grossi said. "What really worries me is that unlike Fukushima (in Japan), where you had mother nature to blame, now it would be us."

-ABC News' Aaron Katersky and Robert Zepeda