As US representatives headed to Moscow after talks with Kiev negotiators, European Union (EU) foreign policy chief Kaja Kallas said on Monday that this week could be crucial in ending the war in Ukraine.

"This could be a crucial week for diplomacy. We heard yesterday that the negotiations in America were difficult but productive. We don't know the results yet, but I will be speaking to the Ukrainian defense and foreign ministers today," Kallas said at a meeting of EU defense ministers.

As Washington seeks to end the Kremlin's war on neighboring Ukraine, U.S. President Donald Trump's envoy Steve Witkoff will travel to Russia on Monday. Witkoff's visit, which includes talks with Russian President Vladimir Putin on Tuesday, follows key talks in Florida on Sunday between senior Trump administration officials and a Ukrainian delegation.

Kallas avoided answering a question about whether she believed the United States would be able to negotiate a good deal, suggesting instead that it would be better if Europe, which Washington has largely left aside, were involved in diplomatic initiatives related to Ukraine.

"The Ukrainians are alone there. If they were together with the Europeans, they would certainly be much stronger. But I believe that the Ukrainians will defend themselves," said Kallas.

Meanwhile, Europe, she said, must focus on"making Ukraine as strong as possible so they can defend themselves in this very difficult time."

The EU is currently debating a plan to provide Ukraine with a massive €140 billion loan financed by frozen Russian assets, but the proposal is opposed by key player Belgium.

Kallas noted that the bloc is determined to achieve"a result on Ukraine financing" at the leaders' summit on December 18, and stressed that"we do not intend to leave" without reaching an agreement.