European leaders are racing to try to figure out how to fill a potential void — in Ukraine and in continental security as a whole — as President Trump’s White House talks of dialing back American support and troop numbers in Europe.

As the war in Ukraine marks its third anniversary and the threat of an emboldened Russia looms, top European officials are flocking to Kyiv on Monday in a show of solidarity.

At the same time, European foreign ministers will be meeting in Brussels, where they are expected to debate how much to send to Kyiv in their next support package.

That plan started with figures in the single digits but could end up totaling more than 20 billion euros, according to two people familiar with the discussions who spoke on the condition of anonymity to discuss internal deliberations. The ministers are also expected to approve a fresh package of sanctions on Russia, which may not please the White House as Mr. Trump draws closer to President Vladimir V. Putin.

More broadly, European leaders are also contemplating the prospect of putting troops on the ground in Ukraine as some sort of peacekeeping or “reassurance” force, officials say. Additionally, they are discussing how to ramp up military spending more generally, with a blueprint for the future of European defense expected to be proposed by the European Commission in mid-March.

And given the turmoil and uncertainty around the American commitment to Europe, António Costa, the president of the European Council, announced on Sunday night that he would convene a special meeting of European leaders on March 6 to discuss Ukraine and European defense.