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

Ukraine has evacuated about 40,000 people in the last 24 hours, but most of the agreed-upon humanitarian corridors in six cities in Ukraine are still not holding, Ukrainian presidential adviser David Arakhmia said Wednesday.

Officials had hoped to move at least 100,000 women and children out of the country in that time frame but were not able to because the corridors were not clear, he said.

The northern city of Sumy was again the only place where a substantial evacuation was able to take place, with local authorities estimating 20,000 people left Wednesday.

An evacuation column was also able to bring people out of Energodar, the city that is home to the nuclear power station attacked last week, taking them to Zaporizhzhia.

An evacuation attempt from the besieged city of Mariupol again had to be abandoned, while in the region north of Kyiv, 50 buses attempting to evacuate people via a new humanitarian corridor became trapped after Russia forces blocked them. It was unclear whether those buses were eventually permitted to continue on to Kyiv. Some people, including 50 children, were evacuated today.

About 250 people were evacuated from the northeastern town of Izyum near Kharkiv, the advisor said.

Ukrainian President Volodymyr Zelenskyy said humanitarian corridors have again been agreed upon with temporary cease-fires for Thursday.

Russia could use chemical or biological warfare as it escalates its attack on Ukraine, White House press secretary Jen Psaki said in a statement to ABC News Wednesday.

"We should all be on the lookout for Russia to possibly use chemical or biological weapons in Ukraine, or to create a false flag operation using them," Psaki said. "It’s a clear pattern, and no one should fall for it."

Psaki prefaced that statement by noting false claims from Russia, endorsed by China, "about alleged U.S. biological weapons labs and chemical weapons development in Ukraine," calling those accusations "preposterous."

"It’s exactly the kind of disinformation operation we’ve seen repeatedly from the Russians over the years in Ukraine and in other countries around the world, which have been debunked conclusively, and an example of the types of false pretexts we have been warning the Russians would invent," she said.

The U.S. is in "full compliance" with its obligations under the Chemical Weapons Convention and the Biological Weapons Convention and "does not develop or possess such weapons anywhere," Psaki said.

-ABC News' Justin Gomez

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