President Joe Biden arrived in Europe Wednesday on a mission to bolster Western unity and ramp up unprecedented sanctions against Russia over its invasion of Ukraine and brutal bid to upset the post-Cold War balance of power.

Underlining how serious the crisis raging in eastern Europe has become, Biden told reporters as he boarded the Marine One helicopter at the White House that he sees "a real threat" of Russian chemical warfare against Ukraine.

With Russian troops increasingly bogged down, the possibility that President Vladimir Putin could order chemical, biological or even tactical nuclear strikes to subdue Ukraine will be one of the dire scenarios discussed at back-to-back NATO, G7 and EU summits in Brussels on Thursday.

Biden, who is set to give a news conference at NATO headquarters on Thursday, will fly Friday to Poland, which neighbours Ukraine and is now the frontline in what some call a new Cold War. On Saturday he meets President Andrzej Duda before returning to Washington.

The war in Ukraine has redefined Biden's 14-month-old presidency as he pivots from domestic priorities to leading the transatlantic alliance in the most serious crisis in Europe for decades.

US President Joe Biden will press for Western unity and tougher Russia sanctions on his European trip
US President Joe Biden will press for Western unity and tougher Russia sanctions on his European trip AFP / Nicholas Kamm

After four years of Donald Trump, who treated European nations as economic competitors and scorned the traditional US role as senior partner in NATO, Biden has put the accent on unity.

White House National Security Advisor Jake Sullivan told reporters travelling with Biden to Brussels that "what we'd 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 said that economic sanctions, imposed by a global network of Western allies to cripple Russia's finances, will be deepened in the summits.

"We, the United States, will announce a package of sanctions designations tomorrow that relate both to political figures, oligarchs... as well as entities," Sullivan said.

Western partners were also looking to "coordinate on sanctions enforcement so that Russian efforts to evade the sanctions can be dealt with effectively".

US National Security Adviser Jake Sullivan warned of 'hard days ahead' for Ukraine
US National Security Adviser Jake Sullivan warned of 'hard days ahead' for Ukraine AFP / Nicholas Kamm

In comments on Tuesday, Sullivan noted Ukraine's military resilience, which has been assisted by large-scale Western arms shipments, but he warned against any presumption of a quick end to the war or a climbdown by Moscow.

"There will be hard days ahead in Ukraine, hardest for the Ukrainian troops on the frontlines and the civilians under Russian bombardment," Sullivan said. "This war will not end easily or rapidly."

If the conflict does drag on, Russia's ability to weather military losses and the crushing Western economic sanctions may partly depend on the position taken by its authoritarian partner China.

Beijing has refused to condemn Putin's war, but Washington hopes at least to dissuade China from actively helping the Kremlin, either by bailing out the Russian economy or sending weapons.

Sullivan said there was no sign that China was providing military assistance in the wake of a nearly two-hour phone call between President Xi Jinping and Biden last Friday.

"This is something we are monitoring closely," he said.

"The president will certainly consult on the question of China's potential participation in the conflict of Ukraine while he's in Brussels. He'll do so at NATO," Sullivan said.

"He'll also do so when he addresses the 27 leaders of the European Union, because on April 1st, the European Union is having a summit with China," he added.

"We believe we're very much on the same page with our European partners."