By Gabriela Baczynska and Andrew Gray

BRUSSELS (Reuters) - European Union heavyweights are set for a showdown with Hungary this week over giving Ukraine billions of euros in aid and the chance to start membership negotiations, both key objectives for Kyiv as its war with Russia stalls.   

European Union leaders will meet for a summit in Brussels on Thursday and Friday to decide on proposals to grant 50 billion euros of economic support to Kyiv, assign a further 20 billion euros to Ukraine's military and launch accession talks.

Securing fresh financial assistance from Europe is critical as doubts mount over future U.S. support for Kyiv, which relies on Western financial aid for its war with Russia.

    But Hungarian Prime Minister Viktor Orban, who boasts about his ties with Russian President Vladimir Putin, has threatened to veto the aid and enlargement talks at the Dec.14-15 summit.

    All three decisions - as well as a fourth one on what would be the EU's 12 package of sanctions against Russia since the invasion begun in February, 2022 - require unanimous backing of all the bloc's 27 countries.

    "We are in a key moment," a senior EU official said, in referring to a stalemate on the battlefield and the U.S. Congress not having approved President Joe Biden's $60 billion aid package for Ukraine.

"It's very important that... the European Union will show clear and full support to Ukraine," the official said under condition of anonymity. "That message wouldn't be only for Moscow, it would also be a message for Washington, it would also be a message for Kyiv."

   Europe's own credibility is also at stake with the bloc having previously vowed to stand by Ukraine as long as it takes.

"We count on positive decisions," Ukraine President Volodymyr Zelenskiy said on Sunday evening. "Europe must defend its values and unity decisively."

    WHAT ORBAN WANTS

    Orban is not new to causing a stir in the European Union.

    Hungary has watered down sanctions against Russia and last December vetoed a deal to grant Ukraine 18 billion euros in 2023.

It eventually allowed that assistance through after haggling for days over EU aid to Hungary blocked over concerns of democratic backsliding under Orban.

As the EU finds itself again seeking to win Orban's support for Ukraine, the executive European Commission is expected to unlock Budapest's access to 10 billion euros this week.

    In opposing opening membership negotiations with Kyiv, Orban initially complained about Ukraine's treatment of the country's Hungarian minority. He has since said Ukraine was too corrupt and not ready to join the EU.

    Instead of deciding on new aid to Ukraine, he demanded the bloc hold a "strategic discussion" on its support for Kyiv.

Diplomats said related bids by Georgia and Bosnia to advance their hopes to join the EU - both backed by Orban - would fall through if Hungary vetoes Ukraine.

"Our feeling was that Orban always knew how far he could go and that he would know exactly when it was time to climb down the tree," said a second senior EU diplomat.

Those expecting Orban to budge described a possible compromise throwing the start on negotiations with Ukraine to March under final conditions. Others, however, worry that this time the Hungarian leader may not be persuaded.

Orban will be up against German Chancellor Olaf Scholz, among others, who has said Berlin backs starting negotiations for Kyiv to join the EU someday.

    FINANCIAL AND MILITARY AID

    While Orban may be the loudest critic of extending more support to Ukraine, a ruling last month by Germany's constitutional court further complicated the EU talks by blowing a massive hole in its richest member's budget.

Should Hungary veto assigning 50 billion euros to Ukraine through the bloc's shared budget, the 26 other EU countries could extend their contributions bilaterally to Kyiv - a more complicated and expensive way.

    Uncertainty also hangs over the future of the EU's military aid to Ukraine, where Russia now controls nearly a fifth of territory.

    A proposal to use an EU-run military fund – the European Peace Facility (EPF) – to give Ukraine 20 billion euros in arms and other support over the next four years has run into resistance from Germany.

Some EU members are pushing for the summit at least to pledge five billion euros to Ukraine via EPF next year, a plan EU foreign ministers will discuss on Monday with their Ukrainian counterpart.

    Despite the gloomy outlook, some Brussels diplomats believe the bloc will avoid the worst-case scenario and deliver on some of the promises made to Ukraine.

    "Will it be difficult? Yes. Will it be extremely difficult? Most likely. Will there be blood in the air at some point? Probably," said a second senior EU official. "But I continue to think it's possible to find solutions."