Russia-Ukraine updates: Putin says 'certain positive movements' in negotiations

Pentagon press secretary John Kirby explained in a press briefing Wednesday why accepting Poland’s MIG-29 jets to aid Ukraine is not a good idea, saying that U.S. intelligence has assessed the move is high-risk and that Russia could see it as "escalatory," therefore increasing the prospects of military escalation with NATO.

"We believe the provision of additional fighter aircraft provides little increased capabilities at high risk," Kirby said.

In a telephone call U.S. Secretary of Defense Lloyd Austin had with Polish Defense Minister Mariusz Błaszczakearlier on Wednesday, Austin thanked Błaszczak for the support to Ukraine provided but essentially said the MiG-29 deal was not the optimal way, Kirby told reporters.

"We believe that the gain from transferring those MIG-29 is low," Kirby said, adding that because the Ukrainian Air Force currently has several squadrons of fully mission capable aircraft, adding aircraft to the Ukrainian inventory likely will not significantly change the effectiveness of the Ukrainian Air Force

The U.S. will explore other options to aid Ukraine, including providing them with the weapons and the systems they need most, such as anti-armor and air defense, Kirby said.

"We also believe that there are alternative options that are much better suited to support the Ukrainian military in their fight against Russia," he said. "We will continue to pursue those options."

-ABC News' Luis Martinez

Russian foreign minister Sergey Lavrov is scheduled to meet with Ukraine foreign minister Dmytro Kuleba in Antalya, Turkey, on Thursday, ahead of the start of the Antalya Diplomacy Forum.

This will be the first face-to-face high-level meetings between Russian and Ukrainian officials since the Russian invasion began.

This meeting is a continuation of the negotiating process "and a very important one," Kremlin spokesperson Dmitry Peskov said, according to Russian state-run news agency TASS.

Lavrov is expected to make a speech on Moscow's position on current issues in Antalya, and Turkish Foreign Minister Mevlut Cavusoglu is also scheduled to attend the meeting with Kuleba.

The third round of talks between Russian-Ukrainian delegates was held in Belarus on Monday.

-ABC News' Christine Theodorou

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