President Joe Biden is seeking to reinforce Western unity at a series of summits in Europe this week where new sanctions will be announced against Russia over its war in Ukraine, a top US official said Tuesday.

Biden leaves Wednesday for Brussels a day ahead of summits with NATO and the European Council, then a trip to Poland on Friday for a meeting with President Andrzej Duda the following day.

The intense diplomacy marks a crucial moment in the dangerous standoff between a newly reinvigorated transatlantic alliance and President Vladimir Putin, who a month ago sent the Russian army pouring into pro-Western Ukraine to try and upend the balance of European power.

White House National Security Advisor Jake Sullivan told reporters that Biden will seek to "reinforce the incredible unity we built with allies and partners."

He also said that unprecedented economic sanctions aimed at crippling Russia's finances would be deepened.

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

A further package will be "rolled out in conjunction with our allies on Thursday," Sullivan said. He gave no details, other than to say the package "will focus not just on adding new sanctions but on ensuring that there is joint effort to crack down on evasion on sanctions."

Another big topic looming for the Western alliance is how to manage China, the world's second biggest economy and an authoritarian partner of Russia.

Beijing has refused to condemn Putin's war or support Western sanctions. So far, Washington's main goal has been to ensure the Chinese do not actively intervene on the Kremlin's side, either by bailing out the Russian economy or even 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.

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

"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."

Although Western-armed Ukrainian forces are successfully resisting much of the Russian onslaught, Sullivan 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," he said. "This war will not end easily or rapidly."

In a wrinkle for Biden's travel plans, his chief spokeswoman, Jen Psaki, will have to stay home because of a positive Covid-19 test announced Tuesday.

The White House press secretary said she had held two socially-distanced meetings with Biden, who is vaccinated, a day earlier.

However, "the president tested negative today via PCR test," Psaki said in a statement, adding that Biden, 79, is "not considered a close contact" as defined by the top US health agency.