President Joe Biden leaves Wednesday for Europe on a mission to bolster Western unity, and ramp up unprecedented sanctions against Russia for invading Ukraine and attempting to upset the post-Cold War balance of power.

The conflict with Russian President Vladimir Putin is redefining Biden's 14-month old presidency as he pivots from domestic woes to leading the transatlantic alliance in the most serious crisis in Europe for decades.

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. At back-to-back summits in Brussels on Thursday, he'll be pushing for more.

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

Sullivan also said that economic sanctions, imposed by a global network of Western allies to cripple Russia's finances, will be deepened.

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."

On Thursday, Biden will attend summits with NATO, the G7 group and the European Council. He flies Friday to Poland, which neighbors Ukraine and is now the frontline in what some call a new Cold War, and on Saturday he meets President Andrzej Duda.

The intense diplomacy marks a crucial moment in the dangerous standoff with Putin, who seeks to force Ukraine from its pro-Western path. After a month of fighting, Russian forces have laid waste to swaths of the country but have made few significant military gains, while Ukraine continues to resist.

China question 

Sullivan noted Ukraine's success, 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."

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.