Hungarian Prime Minister Viktor Orban
Hungarian Prime Minister Viktor Orban AFP

EU leaders will face off against Hungarian Prime Minister Viktor Orban on Thursday as he threatens to veto a massive aid package and a start to membership talks for Ukraine.

The crunch summit in Brussels -- which diplomats fear could drag on longer than the two days planned -- comes as fears mount over Western backing for Ukraine nearly two years into Russia's war.

Kyiv is desperately seeking to improve the narrative after President Volodymyr Zelensky failed in Washington to win over Republican lawmakers blocking support from the United States.

But Orban -- Russia's closest ally in the European Union -- stands in the way of Ukraine's hopes for 50 billion euros ($54 billion) in financial aid and progress towards its goal of one day joining the bloc.

Critics have accused the Hungarian leader of holding Kyiv's survival hostage in a bid to force Brussels to release billions of euros of EU funds frozen over a rule of law dispute.

In what some saw as a last-minute concession the European Commission, the EU's executive, agreed on Wednesday to unblock 10 billion euros of that cash.

But 21 billion euros still remain out of Orban's grasp and it was far from clear that the gesture would avert a damaging dispute at the summit.

The right-wing veteran warned opening accession talks with Ukraine would be a "terrible mistake" and that he would not budge.

Zelensky countered that Orban had "no reason" to block Kyiv from moving towards EU membership and said his country could not beat Russia without more Western support.

The stand-off sets the stage for hours of arm twisting in Brussels as fellow leaders will try to hash out a deal to get Orban to back down.

Russia fired a fresh wave of missiles at Kyiv on Wednesday, wounding dozens of people in its most damaging attack on the capital in months.

Another strike early on Thursday wounded another 11 in the southern Odesa region, emergency services said.

"I'm not that optimistic, having talked to Viktor Orban about these issues," Estonia's Prime Minister Kaja Kallas said on the eve of the summit.

The Baltic leader said the focus was on getting the "plan A" across the line rather than the EU's other 26 countries having to find the money for Kyiv.

"If that is absolutely a no-go, if we have sat here for already very many days and Christmas is coming, maybe even New Year already, then we'll think about a plan B," she said.

European Council chief Charles Michel, whose job it is to corral the leaders towards an agreement, insisted work was going on "day and night" to try to thrash out a deal.

"The most important thing for us as Europe is to support effectively Ukraine," said newly elected Polish Prime Minister Donald Tusk.

"I can't accept any kind of this, you know, mood of apathy or fatigue.

"We are talking not just about Ukraine and the war and the Russian aggression, we are talking about our future," said Tusk, himself a former EU Council chief and summit host.

Tusk's Dutch counterpart Mark Rutte agreed that it was "vital that we agree on the 50 billion for Ukraine".

"They are fighting our fight. And this has to be this week. So I will work tirelessly with all my colleagues to get it adopted," he said.

The summit also has to consider the membership dreams of Moldova and Georgia, two other ex-Soviet states menaced by Moscow.

Moldova is hoping leaders will greenlight it starting talks to join as well. Georgia is aiming to reach the step behind that -- becoming an official candidate for membership.

Even if Ukraine and Moldova are granted talks, the protracted process of joining the EU will involve years -- if not decades -- of major reforms.