BRUSSELS—The European Union for the first time will finance the purchase and delivery of weapons, in a show of support for Ukraine against Russia’s invasion, European Commission President Ursula von der Leyen said Sunday.

The EU also will extend to Belarus sanctions similar to those it imposed on Russia, ban the entry of Russian planes into EU airspace and ban from the 27-country bloc Russian state-owned media outlets Russia Today and Sputnik, as well as their subsidiaries.

Ms. von der Leyen said the EU's decision to finance arms purchases was “a watershed moment.” The EU traces its roots to post-World War II peace efforts and began as an economic alliance.

In closing its airspace, the EU banned not only commercial airliners but any plane owned, registered or controlled by Russians, Ms. von der Leyen said.

“Our airspace will be closed to every Russian plane–and that includes the private jets of oligarchs,” Ms. von der Leyen said.

In announcing sanctions on Belarus, Ms. Von der Leyen said that its leader, Aleksander Lukashenko, “is complicit in this vicious attack against Ukraine” and so would face bans on exports of products including mineral fuels, tobacco, wood, cement, iron and steel. Export restrictions already imposed on Russia also will be extended to Belarus, she said.

In funding arms purchases for Ukraine, the EU won't buy weapons directly but reimburse purchases or transfers from its member countries to Ukraine’s military. The decision to fund lethal weapons marks a sharp turn in EU policy, which until now has largely precluded deadly engagement in conflicts.

“Another taboo has fallen,” said EU foreign-policy chief Josep Borrell, standing alongside Ms. von der Leyen. “We are doing it because this war requires our engagement in order to support the Ukrainian army.”

The funding will come from the European Peace Facility, a financing instrument established by the bloc last March aimed at preventing conflicts and strengthening international security. Able to provide several hundred million dollars annually, it was established outside the EU’s normal budget, in part because inclusion could have complicated arms purchases.

Mr. Borrell said he had directed the EU’s military staff -- which until now has engaged in activities around preventing conflicts -- to coordinate Ukrainian military requests with member states for maximum efficiency.