President Volodymyr Zelenskyy hailed a "victory" for Ukraine and the European continent on Thursday after European Union leaders agreed to open membership talks for Ukraine and Moldova despite months of opposition from Hungary about Kyiv joining.

The decision announced by European Council President Charles Michel on the first day of a summit in Brussels is a much-needed morale boost for Kyiv, which fears vital Western support has been waning as its war with Russia rages on with no end in sight.

"I thank everyone who worked for this to happen and everyone who helped. I congratulate every Ukrainian on this day. ... History is made by those who don't get tired of fighting for freedom," Zelenskiy wrote in a post on social media platform X.

In a separate post on X, he added: "This is a victory for Ukraine. A victory for all of Europe. A victory that motivates, inspires, and strengthens."

It was not immediately clear what the fate was of a four-year, $50 billion euro aid package that Kyiv hopes will also be agreed by EU leaders at the summit this week.

Ukraine announced its wartime bid to join the EU days after Russia launched its full-scale invasion in February 2022.

"This is an extremely important milestone on our common path to the unification of Europe... When we started it, no one believed we'd succeed. But we didn't care," said Andriy Yermak, Zelenskyy's chief of staff.

Kyiv has been racing to agree and adopt legislative reforms in recent weeks to meet the criteria for talks to be launched, but had faced staunch opposition from Hungarian Prime Minister Viktor Orban.

Ukrainian Foreign Minister Dmytro Kuleba said: "What a historic day. One emotion dominates: everything was not in vain."

Deputy Prime Minister Olha Stefanishyna said: "(This) became possible only thanks to the strong will of all Ukrainians, our soldiers standing on the frontline."

Russia, which has occupied more than a sixth of Ukraine's territory, is a fierce opponent of Ukraine's push to join Western institutions like the NATO military alliance.

Moscow's troops seized and annexed the Ukrainian peninsula of Crimea in 2014 after mass protests in Kyiv that toppled a Russian-backed leader who had abandoned a push to sign an association agreement with the European Union.

Moldova, which lies between Ukraine and Romania, hailed the EU decision to open formal accession talks with it. Pro-Western President Maia Sandu said Moldova would rise to the challenge and was committed to the "hard work" that lay ahead.

"Moldova turns a new page today with the EU's go-ahead for accession talks. We're feeling Europe's warm embrace today. Thank you for your support and faith in our journey," Sandu wrote on X.