Hungary's veto came at the European Council summit in Brussels on Thursday night, public broadcaster Polish Radio's IAR news agency reported.
Earlier in the day, EU leaders, including Polish Prime Minister Donald Tusk, decided to start accession negotiations with Ukraine and Moldova, and approved a new round of sanctions against Russia, among other measures.
But the summit failed to agree on financial aid for Kyiv. Hungary's Prime Minister Viktor Orban said on social media that he had lodged a "veto for the extra money to Ukraine."
Orban added that EU leaders would revisit the issue next year "after proper preparation."
European Council President Charles Michel told reporters the decision had been postponed to an extraordinary EU summit in January.
The proposed financial package for Kyiv consists of EUR 33 billion in loans and EUR 17 billion in grants, Polish state news agency PAP reported.
EU reaffirms support for Ukraine 'for as long as it takes'
In their summit conclusions on Ukraine, EU leaders renewed their "resolute condemnation of Russia’s war of aggression against Ukraine."
They also reaffirmed the bloc's "unwavering support for Ukraine’s independence, sovereignty and territorial integrity within its internationally recognised borders and its inherent right of self-defence against the Russian aggression."
EU leaders further stated: "The European Council recalls its previous conclusions and confirms the European Union’s unwavering commitment to continue to provide strong political, financial, economic, humanitarian, military and diplomatic support to Ukraine and its people for as long as it takes."
The EU summit in Brussels concludes on Friday.
Friday is day 660 of Russia's war on Ukraine.
Russia invaded Ukraine on February 24, 2022, launching the largest military campaign in Europe since World War II.
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Source: IAR, PAP, Reuters, Politico