With its economy slipping towards recession and Budapest already seeking an international funding deal, unexpected extra liabilities pose a huge threat to Hungary's deficit target.

Malev's collapse and a move by mobile phone company Nokia (NOK1V.HE) to scale back manufacturing in Hungary are already expected to cost the country 5,000 jobs.

Germany's Hochtief (HOTG.DE) and its partners said on Thursday it would take some time to assess any claim and said they would help Hungary in efforts to resume Malev's passenger operations.

A Hochtief spokeswoman also said any realisation of claims depended on the Hungarian government.

Hungary could be liable, in a worst case scenario, for damages worth hundreds of billions of forints due to lower passenger numbers and investments made at the airport.

Hochtief and four other investors bought a 75 percent stake in Budapest airport in 2007 and now own all of it.

Malev accounted for 40 percent of annual turnover at the airport before its collapse last Friday after its planes were held overseas for unpaid debts.

Hungary's Development Ministry said late on Wednesday that under a previous government the Hungarian state had effectively assumed a full guarantee for "everything."

"The ministry is aiming for a reasonable, acceptable compromise," it added.

The government has said a new airline could be formed only as a green-field investment and the prompt entry of rival airlines into the gap left by Malev could affect the viability of any new carrier.

MASSIVE BLOW

Government Commissioner Gyula Budai said on Wednesday that the airport bill could run to 800 billion forints -- the equivalent of nearly 1.5 times the government's 2012 budget deficit target.

The International Monetary Fund, which has shelved informal talks on aid with Budapest due to the government's hard line on controversial reforms, flagged downside risks to its growth estimate for Hungary and said Malev's demise posed new threats.

"There are fiscal risks that stem in particular from Malev because it's a new, recent event, and the situation is really unfolding day by day," Iryna Ivaschenko told a business conference in response to a question.

She said it was too early to assess how this would affect headline budgetary numbers.

Budapest Airport said on Thursday that Malev's collapse stripped it of 1.5 million transit passengers per year, knocking a hole in its financial plans despite rival airlines announcing flights on more than 60 percent of Malev's old routes.

It said the airport, whose owners had committed to investments of a total of 261 million euros by the end of 2011, served nearly 8.9 million passengers last year.

Budget carriers including Ireland's Ryanair (RYA.I), Budapest-based Wizz Air and Air Berlin have announced extra flights from Budapest to fill the gap left by Malev's collapse.

(Additional reporting by Marton Dunai; Editing by David Cowell)

• A Permanent Economic Emergency