* New PM says economy needs tough measures
* Says $37 billion of state loans disappeared
* Puts capital flight at $70 billion in 3 years
By Natalia Zinets and Timothy Heritage
KIEV, Feb 27 (Reuters) - Ukraine's new prime minister said on Thursday loans worth $37 billion had gone missing from state coffers during ousted President Viktor Yanukovich's rule, and warned that unpopular measures were needed to salvage the economy.
With the hryvnia currency in freefall and concerns about the low level of foreign currency reserves growing, Arseny Yatseniuk said the country urgently needed loans from the International Monetary Fund, which is visiting Kiev next week.
The scale of alleged theft implied by Yatseniuk in a speech to parliament was jaw-dropping, even for a population now used to tales of Yanukovich's extravagance and lavish lifestyle, including his luxury residence outside Kiev.
The average salary in Ukraine is around $500 a month.
In addition to the missing $37 billion, Yatseniuk said as much as $70 billion had been sent out of the country during Yanukovich's three-year rule, although he did not make clear how much of this capital flight was illegal.
"I want to report to you - the state treasury has been robbed and is empty," he said before the national assembly voted him in as head of a national unity government.
"Thirty-seven billion dollars of credit received have disappeared in an unknown direction ... (and) the sum of 70 billion dollars was paid out of Ukraine's financial system into off-shore accounts."
At today's rate, $70 billion is equal to about half Ukraine's gross domestic product in 2013.
Only 4.3 billion hryvnia - $430 million - was left in government accounts, Yatseniuk said, although the central bank's foreign currency reserves stand at $15 billion.
Shortly after he spoke, the Swiss government said it would order banks in the country to freeze any funds found to be linked to Yanukovich.
In Ukraine, the situation was so grave that there was no choice but to take "extraordinarily unpopular measures," Yatseniuk said, looking around the solemn faces in parliament as he listed the depth to which the economy has sunk.
With prospects for foreign aid growing, Ukraine's dollar bond maturing in 2017 rose 1.5 points on Thursday to trade at 91.65 cents in the dollar, while bonds issued by state energy company Naftogaz maturing this September rose 1 point.
The cost of insuring Ukraine's debt also slid as approval of the interim government opened the possibility of negotiating a deal with the IMF.
An IMF fact-finding team is to visit Ukraine in the coming days, in response for Ukraine's request for help, Managing Director Christine Lagarde said.
However, the June 2014 government bond weakened, signalling that the possibility of a short-term default is still worrying investors.
The hyrvnia traded as low as 11.0 to the dollar and markets were signalling more depreciation, with 6-month currency forwards pricing it at around 12 per dollar.
A former economy and foreign minister, and ex-deputy head of the central bank, Yatseniuk's immediate task is to draw up an anti-crisis plan and secure international aid to prevent a default and shore up the hyrvnia.
Finance Minister Oleksander Shlapak suggested Ukraine wanted a $15-billion aid package from the IMF.
Acting President Oleksander Turchinov had said at the weekend that the country of 46 million needed more than $30 billion over two years. The previous government had said last year that $20 billion was needed.
The fate of a $15-billion bailout package agreed with Russia in December is not clear, with Moscow having so far released only $3 billion of the sum promised. Uncertainty also hangs over an agreement that reduced the amount Ukraine pays Russia for gas.
A previous IMF financial package, worth $15.5 billion, was frozen in 2011 after the Ukrainian government reneged on the terms, balking at putting up energy prices, a move that would have been unpopular with voters.
Shlapak and Yatseniuk said IMF money would help to stabilise the hyrvnia. The currency has been falling for weeks - losing more than a fifth of its value - but the drop has accelerated since parliament stripped Yanukovich of his powers and he fled the capital. His whereabouts now are not known.
Tatyana Orlova, a strategist at RBS in London, said the hryvnia could weaken to 16 per dollar if nothing were done and added: "A big devaluation is not unjustified for the economy."
Neuberger Berman said in a research note that many hurdles lay ahead for the Ukrainian economy. "We are also unsettled by the seemingly endless increases in headline figures needed for Ukraine to avoid default in two years," it said. "To us, this trend fosters doubt as to the commitment of future leadership to undertake much-needed reforms."