Volodymyr Zelenskyy and Belgium's Prime Minister Alexander De Croo have signed a security pact in Brussels.
Mr De Croo said the pact includes the delivery of 30 F-16 fighter jets to Kyiv and at least €977m (£832m) in Belgian military aid to Ukraine this year.
Separately, the Netherlands had pledged to deliver parts of a Patriot air defence system to Ukraine, its defence minister has said.
It is also talking to other countries in order to assemble a complete system, Dutch defence minister Kajsa Ollongren said.
"With our offer, and if other potential partner countries are able to deliver parts and ammunition, we would be able to supply at least one complete system to Ukraine in the short term," she said.
