Hungarian Prime Minister Viktor Orban said on Friday that "very difficult questions" would need to be answered before the European Union could even start membership talks with Ukraine.
EU countries are due to decide in December whether to allow Ukraine to begin accession negotiations, which would require the unanimous backing of all 27 members. Diplomats have said Hungary may be an obstacle.
"We cannot avoid the question - when during the autumn we will have negotiations in Brussels about the future of Ukraine - whether we can actually seriously consider membership for a country, to start accession talks with a country that is at war," Orban told state radio.
"We don't know how big the territory of this country is, as the war is still ongoing, we don't know how big its population is as they are fleeing ... to admit a country to the EU without knowing its parameters, this would be unprecedented.
"So I think we need to answer very long and difficult questions until we get to actually deciding about the start of accession talks," he said.
Commenting on Orban's remarks, Ukraine's foreign ministry said it was positive "that the Hungarian Prime Minister is concerned about Ukraine's accession to the European Union".
"We would like to inform that Ukraine has not changed its territory within its internationally recognised borders," the ministry added.
Orban cultivates warmer ties with Moscow than many of his EU peers and has repeatedly locked horns with Kyiv, including over the right of ethnic Hungarians to learn in their native language after Ukraine passed a 2017 law restricting the use of minority languages in schools.
Orban told parliament on Monday that Hungary will not support Ukraine on any issue in international affairs until the language rights of ethnic Hungarians there are restored.
Hungary is a member of NATO and opposes Russia's invasion of Ukraine. Hungary has refused to ship weapons to Ukraine but has welcomed Ukrainian refugees fleeing the war.
