Russia-Ukraine updates: 2 US veterans who joined Ukrainian forces missing

Russian President Vladimir Putin hinted at the possibility of nuclear warfare during his Wednesday address to the council of legislators.

"If someone from outside moves to interfere in the current developments, they should know that they will indeed create strategic threats to Russia, which are unacceptable to us, and they should know that our response to encounter assaults will be instant, it will be quick," Putin said, according to Russian state media.

Putin claimed Russia's response to strategic threats from outside Ukraine would be “immediate."

“We have all the tools to do it, tools that others can't boast of at the moment, but as for us, we won't be boasting,” Putin said.

Putin said that Russia is prepared to use those “tools” if “the need arises," adding that he “would like everyone to be aware of it.” A nuclear attack has been on the table since the onset of the “special military operation” in Ukraine, Putin said. He had ordered his nuclear forces to be put on high alert on Feb 27.

Putin's remarks came as Pentagon press secretary John Kirby announced that “more than half” of the 90 howitzers the U.S. agreed to send to Ukraine were now in the country, adding that around 50 Ukrainian troops have already been trained to operate the weapons.

“We finished up earlier this week, the first tranche of more than 50 trainers that are going to go in and train their teammates,” Kirby said during a press briefing on Wednesday, a moment later adding, “But there was another tranche of more than 50 that we're going to go through training in the same location outside Ukraine.”

The U.S. Department of Defense on Wednesday tweeted pictures of more howitzers “bound for Ukraine” that were being loaded onto US Air Force aircraft. Additional training opportunities on Howitzers and other weapons systems were also being explored, Kirby said.

As U.S. weapons head to Ukraine, Russia is increasing the pace of its offensive in almost all directions, the General Staff of the Ukrainian Armed Forces said on Thursday.

The U.S. is considering the legal aspects of officially listing Russia as a state-sponsor of terrorism, Secretary of State Anthony Blinken told lawmakers on Wednesday. Officials said they haven't yet determined whether Russia's actions meet the legal standard required for the designation, Blinken said.

The designation, called for by Ukrainian President Volodymyr Zelenskyy and Foreign Minister Dmytro Kuleba, would further cripple Russia's trade potential, including bans on defense exports and limits on foreign aid.

-ABC News’ Edward Szekeres, Uliana Lototska and Fidel Pavlenko

President Joe Biden's supplemental budget request to secure more funding for Ukraine could be sent to Capitol Hill as soon as Thursday, according to White House press secretary Jen Psaki.

"In terms of the length or the size, I don't have a number for you at this point," Psaki told reporters at Wednesday's briefing. "But there is plans for this to be a proposal to go through the fiscal year, and it will include -- as our past packages have included -- security or military assistance, humanitarian economic assistance."

Psaki promised that Biden would stick to his word of delivering the supplemental request before the end of the week.

Last Thursday, Biden announced that he would make this request of Congress while announcing the latest $800 million aid package for Ukraine.

“I've almost exhausted the drawdown authority I have that Congress authorized for Ukraine in a bipartisan spending bill last month. In order to sustain Ukraine for the duration of this fight, next week I'm going to have to be sending to Congress a supplemental budget request to keep weapons and ammunition flowing without interruption,” Biden said last week. “My hope and expectation is that Congress would move and act quickly.”

-ABC News' Molly Nagle

While U.S. diplomats began returning to Lviv for day trips on Tuesday, Secretary of State Antony Blinken said Wednesday that he hopes to move toward reopening the U.S Embassy in Kyiv in the "next few weeks."

Blinken appeared on Capitol Hill Wednesday for his second of three days of testimony about the Biden administration's budget request.

Blinken said the administration will put forward a request for supplemental funding in the "next couple days" after President Joe Biden exhausted the funding in his presidential drawdown authority to provide weapons and other military aid immediately to Ukraine.

That "robust" assistance request will include funding for aid to Ukraine and other U.S. partners and allies and for a functioning U.S. Embassy in Kyiv, he said.

-ABC News' Conor Finnegan

Microsoft has released a detailed report of what it says are "destructive" Russian cyberattacks on Ukraine, which the company says seem "strongly correlated and sometimes directly timed with its kinetic military operations."

"For example, a Russian actor launched cyberattacks against a major broadcasting company on March 1st, the same day the Russian military announced its intention to destroy Ukrainian 'disinformation' targets and directed a missile strike against a TV tower in Kyiv," Microsoft said. "On March 13th, during the third week of the invasion, a separate Russian actor stole data from a nuclear safety organization weeks after Russian military units began capturing nuclear power plants sparking concerns about radiation exposure and catastrophic accidents. “

Microsoft said it has observed nearly 40 attacks "targeting hundreds of systems."

The company said "32% of destructive attacks directly targeted Ukrainian government organizations" while "more than 40% of destructive attacks were aimed at organizations in critical infrastructure sectors that could have negative second-order effects on the Ukrainian government, military, economy and people."

-ABC News' Cindy Smith