President Joe Biden escorted by Col. Matthew Jones, Commander of the 89th Airlift Wing, walks to board Air Force One at Andrews Air Force Base, Md., Wednesday, March 23, 2022. AP Photo/Gemunu Amarasinghe

BRUSSELS — As war rages on in Ukraine, President Biden will huddle with key allies in Brussels and Warsaw this week to talk through plans for imposing punishing new sanctions on Russia and dealing with an extraordinary humanitarian crisis, while developing a consensus on how they would respond if Russia were to launch a cyber, chemical or even nuclear attack.

Biden arrived in Brussels on Wednesday for a four-day trip that will test his ability to navigate Europe’s worst crisis since World War II ended in 1945. There are fears that Russia could use chemical or nuclear weapons as its invasion becomes bogged down in the face of logistical problems and fierce Ukrainian resistance.

“I think it’s a real threat,” Biden said of the possibility of Russia deploying chemical weapons. He spoke during a brief exchange with reporters at the White House before his departure.

Humanitarian challenges are growing as well. Millions of refugees have fled the fighting, mostly by crossing the border into Poland, and the war has jeopardized Ukraine’s wheat and barley harvests, raising the possibility of rising hunger in impoverished areas around the globe.

As Biden made his way to Brussels, his top diplomat announced he had made a formal determination that Russian troops have committed war crimes in Ukraine.

Secretary of State Antony Blinken, traveling with Biden, said in a statement the assessment was made on a “careful review” of public and intelligence sources since Russia launched its invasion of Ukraine a month ago. He said the U.S. would share that information with allies, partners and international institutions tasked with investigating allegations of war crimes and crimes against humanity.

“We’ve seen numerous credible reports of indiscriminate attacks and attacks deliberately targeting civilians, as well as other atrocities. Russia’s forces have destroyed apartment buildings, schools, hospitals, critical infrastructure, civilian vehicles, shopping centers and ambulances, leaving thousands of innocent civilians killed or wounded,” Blinken said.

Jake Sullivan, Biden’s national security adviser, said the president would coordinate with allies on military assistance for Ukraine and new sanctions on Russia during meetings Thursday with NATO officials, Group of Seven leaders and European Union allies.

At NATO, Biden and fellow leaders will “set out a longer term game plan” for what forces and capabilities are going to be required for the alliance’s eastern flank countries, Sullivan said. Leaders of several Eastern European NATO members have pressed for a greater U.S. and NATO presence in their backyards in the aftermath of the Ukraine invasion.

NATO Secretary-General Jens Stoltenberg said four new battlegroups, which usually number between 1,000 and 1,500 troops, are being temporarily set up in Hungary, Slovakia, Romania and Bulgaria. A permanent force posture is expected to be formally announced at the next NATO summit in Madrid in June, Sullivan said.

European Union nations on Wednesday also signed off on another 500 million euros in military aid for Ukraine. EU foreign affairs chief Josep Borrell called the doubling of the EU’s military aid since the Feb. 24 beginning of the war “another sign of the EU’s support to the Ukrainian armed forces to defend their territory and their population.”

At the meeting of the Group of Seven, leaders from the bloc of wealthy, industrialized nations are expected to unveil a new initiative to coordinate sanctions enforcement and unveil additional sanctions against Russian officials.

One new sanctions option that Biden is looking at is to target hundreds of members of the Russian State Duma, the lower house of parliament, according to a U.S. official who spoke on the condition of anonymity to discuss the potential move. The official added that a final decision hasn’t been made and that the new sanctions would be rolled out in coordination with Western allies.

Sullivan said additional Russian oligarchs and political figures would be among those designated in the sanctions unveiled Thursday.

Earlier this week, Biden warned that Russia could be planning cyberattacks that would affect U.S. companies. The U.S. president also spoke to Chinese President Xi Jinping on Friday to warn him against backing Russia with military or financial assistance.

U.S. concerns about China are expected to be on the agenda when Biden attends a meeting of the European Council, where he will also discuss the worsening refugee and humanitarian crises that have developed over the past month. The European Union is scheduled to hold a summit with China on April 1.

Central to the president’s agenda during his time in Europe is making certain that the U.S. and its allies remain on the same page.

“What we would like to hear is that the resolve and unity that we’ve seen for the past month will endure for as long as it takes,” Sullivan told reporters on Air Force One en route to Brussels.

Sullivan also said the United States is looking for ways to “surge” supplies of liquified natural gas to Europe to help make up for supply disruptions. The European Union imports nearly all of the natural gas needed to generate electricity, heat homes and supply industry, with Russia supplying nearly half of EU gas and a quarter of its oil.

Before the trip, Sullivan said Putin’s references to nuclear weapons at the beginning of the conflict are “something that we do have to be concerned about,” adding that Biden would be talking with allies about “potential responses” if the Russian leader takes that step.

Sullivan’s description of Biden’s trip was another sign that the crisis is entering a new and uncertain phase.

After the initial invasion failed to topple Ukraine’s government, the war has become a grinding endeavor for Putin, who is relying on airstrikes and artillery that are devastating civilian communities. Negotiations between Ukraine and Russia have not produced a cease-fire or a path to ending the conflict, and the U.S. continues to rush weapons like anti-tank missiles to Ukrainian forces.

“This is one of those decisive moments for an American leader that defines their legacy internationally,” said Timothy Naftali, a presidential historian at New York University.

Americans also are increasingly viewing the crisis as one that will require economic sacrifice.

A new poll from The Associated Press-NORC Center for Public Affairs Research finds a majority of Americans say they’re willing to accept damage to the economy if it helps to stop Putin’s invasion. Forty percent now say the U.S. should have a “major role,” up from 26% in an AP-NORC poll conducted just before the invasion began.

Forty-six percent say the U.S. should have a “minor role.” The percentage who think the U.S. shouldn’t be involved at all ticked down from 20% to 13%.

Biden departed for Europe as public health officials took note of a global uptick in COVID-19 cases. Confirmed cases of the virus had been falling steadily worldwide since January but rose again last week, due to the more infectious omicron variant and the suspension of COVID protocols in numerous countries in Europe, North America and elsewhere, the World Health Organization reported on Tuesday.

Biden’s press secretary, Jen Psaki, announced on Tuesday that she would not travel with Biden after testing positive for the virus for the second time in five months. Biden was last tested on Tuesday, according to the White House. Psaki said she had two “socially distanced meetings” with Biden on Monday and that he is not considered a “close contact” under Centers for Disease Control and Prevention guidelines.

Biden travels to Warsaw on Friday to meet Polish officials to discuss the enormous humanitarian strain caused by the Ukrainian refugee crisis. Biden is scheduled to meet with Polish President Andrzej Duda on Saturday.

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Associated Press writer Vanessa Gera in Warsaw, Poland, contributed to this report.