A decision to start talks on Ukraine’s EU accession is on a knife-edge after Hungary said it would not bow to mounting pressure to give the green light. Viktor Orbán’s threat to veto the launch of negotiations is being taken seriously, with Ukraine’s foreign minister warning of “devastating consequences” for his country if the talks are blocked. The 27 EU leaders are due to meet on Thursday and Friday and one diplomat said the mood in Brussels was increasingly bleak. Orbán, the Hungarian prime minister, who boasts about his strong ties to Vladimir Putin, has said he will block the decision on EU enlargement and potentially block continued financial support for Ukraine. On Sunday, he and the Ukrainian president, Volodymyr Zelenskiy, shared a short and apparently intense exchange at the inauguration of Argentina’s new president. Last week, in a defiant move, Orbán declared Ukraine to be “one of the most corrupt countries in the world”, hours after the French president, Emmanuel Macron, had met him to try to persuade him not to veto EU enlargement talks. On Monday, before a meeting of foreign ministers in Brussels, Hungary’s foreign minister, Péter Szijjártó, doubled down on that stance, saying Budapest would not “give in to any pressure”. He wrote on Facebook: “A majority of European politicians want to make such important decisions which are entirely unprepared and lack strategic agreement on the future of Europe. We will not give in to any pressure … irrespective of where that is coming from, from whom, and what kind of blackmail it is or promise.” Arriving for separate talks with the ministers, Ukraine’s foreign minister, Dmytro Kuleba, pleaded with the EU to prevent Hungary from doing damage. Referring to the possibility of opening accession talks as “the mother of all decisions”, he said: “I cannot imagine, I don’t even want to talk about the devastating consequences that will occur shall the [European] Council fail to make this decision.” EU ambassadors in Brussels are working overtime to try to resolve the matter, working on the latest text until 11pm on Sunday. One diplomat said: “It is bleak. It is going to be a very difficult week.” Even meetings with Macron and Spain’s Pedro Sánchez had not seemed to have had an impact on Orbán. “What he wants is money, power and recognition,” the diplomat said. The EU is expected to release €10bn of about €30bn in funds withheld from Hungary on Tuesday after it was argued that Budapest had instituted reforms to ensure the independence of its judicial system as demanded by Brussels. But that money is unlikely to be enough to persuade Hungary to change course this week. The EU is so concerned that it is already working on plans B and C, sources said. In addition to EU enlargement, on the agenda for leaders on Thursday is €50bn of economic support and how to continue military aid for Ukraine’s defence against Russia’s invasion. Council decisions can be blocked by any country using a veto. One scenario being considered by EU leaders to circumvent Hungary’s veto is to use a different legal instrument to get the financial aid to Ukraine using qualified majority voting. Another is to schedule an emergency summit in January on enlargement and declare the intention to open negotiations next year as long as Ukraine “continues on the path” of reforms required. The European Commission said Ukraine had met all requirements for negotiations to begin on 8 November with officials pledging to send a team of staffers to Kyiv this Friday night if, as expected back then, the EU leaders endorsed the commission’s recommendation for Ukraine and Moldova to begin talks. Securing new financial assistance from Europe is critical for Zelenskiy at a time when united support in the US is faltering, with Republicans trying to tie aid to funding for US border security. Zelenskiy is due to visit Washington on Tuesday to meet Joe Biden and Republican senators.