House Speaker Nancy Pelosi, D-Calif., said Ukrainian President Volodymyr Zelenskyy reiterated that his country needs aircraft in order to continue its resistance from the Russian invasion.

"He wants planes. He wants anti-missile weaponry. He wants armored cars ... tanks," Pelosi said of the 45-minute call on Wednesday. "He wants the planes, the planes, the planes."

It was not clear whether Pelosi was aware of the Pentagon’s statement rejecting a plan from Poland to provide Soviet jets to Ukraine in exchange for newer American replacements. She also continued to dim the prospects of the U.S. instituting a "no-fly" zone.

"He also said that we're going to need everybody's help rebuilding Ukraine," Pelosi said.

-ABC News' Lalee Ibssa, Benjamin Siegel and Rachel Scott

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

The Russian forces charging toward Kyiv haven't made progress in the last day as they face Ukrainian resistance and low food and gas supply, a senior U.S. defense official told reporters Tuesday.

However, it could be a deliberate pause, the official said. "Part of the stall could be ... as a result of their own self-determined sort of pause in operations -- that they are possibly regrouping, rethinking, reevaluating," the official said.

The U.S. believes Russian forces "have committed now more than 80% of what was their pre-staged combat power," the official added.

The official said some Russian soldiers weren't told they were going into combat. The official said "not all of them were apparently fully trained and prepared."

The strong Ukrainian resistance has also hurt morale, according to the official.

Russia has now launched more than 400 missiles on Ukraine, the official said. The U.S. believes Russia has launchers that could be used for thermobaric weapons, but cannot confirm their use, the official said.

Russian forces are making the most progress in the south. Russians are attacking Kherson in south Ukraine, which "appears very much to be contested city at this point," the official said.

Russians are also approaching Mariupol in southeastern Ukraine, and while they haven't yet entered the city, "they are close enough now that they could attack Mariupol with long range fires," the official said.

Two towns on the path to Mariupol are believed to be occupied by the Russians, according to the official.

The U.S. believes the Russians hope to move north out of Mariupol up to the heavily-contested city of Kharkiv. The official said they believe Russian forces are trying to encircle Kharkiv.

The U.S. official noted that they've seen "certain risk-averse behavior by the Russian military" over the last week.

"Take the amphibious assault, for instance. They put those troops ashore a good 70 kilometers away from Mariupol because they knew Mariupol was going to be defended and they could put them ashore in an uncontested environment. And they still haven't reached Mariupol," the official said.

"They are not necessarily willing to take high risks with their own aircraft and their own pilots," the official said.

"And of course we're seeing that on the ground -- the fairly slow and steady progress that they have made, and you guys are seeing it for yourselves on the ground where ... units are surrendering, sometimes without a fight."

-ABC News' Matt Seyler