Ms. Psaki provided few details of Mr. Biden’s trip and declined to say if he planned to meet with Ukraine’s president, Volodymyr Zelensky, whom he has praised as a hero. She also would not say whether Mr. Biden or other U.S. officials would meet with refugees who have fled the fighting.

“We’re still finalizing the trip at this time,” said Ms. Psaki, adding, “Nothing on the table at this point in time, but the real focus right now is to meet with NATO partners in Brussels.”

The announcement of Mr. Biden’s trip comes after Vice President Kamala Harris visited Poland and Romania, NATO allies along Ukraine’s western borders, last week to express support for Ukraine and suggest that Moscow might need to be held accountable for possible war crimes in the conflict. Mr. Zelensky is expected to request greater economic and military aid when he delivers a virtual address to Congress on Wednesday.

Mr. Biden will join 30 NATO leaders at the special, hastily organized meeting, then plans to attend a previously scheduled European Council summit. Twenty-one of the European Union’s 27 members belong to NATO, and it is possible that close NATO allies like Sweden and Finland will also attend the meeting.

Administration officials said that the White House was exploring whether Mr. Biden might continue his trip to Eastern Europe, to meet some of the refugees flowing out of the country.

Mr. Biden also plans to announce more support for Ukraine’s defense, Ms. Psaki said.

“We are on Day 19,” Ms. Psaki noted of the Russian invasion.

“In addition to their incredible courage and bravery,’’ she said of the Ukrainians, “one of the reasons that they are able to hold back and push back against the Russian military is because of the significant amount of military assistance we have provided.”

Mr. Biden, officials say, wants to go to Europe to make the case that he is succeeding at unifying NATO and other European allies. But European diplomats, concerned that there are no immediate major decisions on the table, are working to come up with a major announcement or initiative that could arise from the extraordinary meeting of the Western alliance.

Mr. Biden’s goal,” Ms. Psaki said, “is to meet face to face with his European counterparts and talk about where we are at this point in the conflict, in the invasion of Ukraine by Russia.”

Ms. Psaki shrugged off Russia’s announcement on Tuesday that it would impose reciprocal sanctions on top U.S. officials, including herself and President Biden. She suggesting, jokingly, that the Kremlin’s announcement might have missed its intended mark. The president, Ms. Psaki said, is a “junior, so they might have sanctioned his dad by mistake.”