The budget reforms that Greece has proposed are not enough to ensure a surplus this year, the IMF's chief economist Olivier Blanchard has told a French financial newspaper Les Echos.
Greece's radical-left government has been locked in tortuous negotiations with the IMF, the European Union and the European Central Bank for the past four months in a bid to unlock €7.2 billion in bailout cash.
Greece was supposed to have a 3% budget surplus this year, but that now looks out of reach, Mr Blanchard said.
"Given the latest estimations show a substantial budget deficit for the time being, credible measures are needed to transform this into a surplus and maintain a surplus in the future," he said.
"Seeing what has been put forward so far, we are still quite far off."
The IMF "is flexible on what Athens needs to do, providing a coherent programme is presented in the end", Blanchard was quoted as saying in an interview on the paper's website.
He said he believed Greece still had structural problems, saying "the pensions system is often too generous and there are still too many civil servants".
"There is a need to look at what structural measures are essential to guarantee sustained medium-term growth."
Mr Blanchard, a Frenchman, said if Greece failed to find a solution with its creditors, then several scenarios were possible, including an exit from the euro, but "everyone hopes to avoid them".
A government spokesman said that Greece will keep on repaying its international creditors for as long as it can, with some €300m due next week and no deal yet in sight.
"To the extent that we are able to pay, we will keep on repaying these obligations," spokesman Gabriel Sakellaridis said.
He added: "It is the government's responsibility to be able to repay all these obligations.... It is also the responsibility of the creditors to be faithful to (their) loan obligations."
He said that Greece and its creditors were still seeking common ground on the key issues of tax, social insurance, labour rights and the size of its budget surplus.
Mr Sakellaridis's remarks came a day after a cabinet minister said Greece had "no money" to make a series of repayments to the International Monetary Fund from 5 June.
"The instalments for the IMF in June are €1.6bn. This money will not be given. There isn't any to be given. This is a known fact," Interior Minister Nikos Voutsis said yesterday.
The Syriza-led government, which was elected in January on an anti-austerity platform, has so far refused to agree to key economic reforms that the creditors want in exchange for the rescue funds.
But with a punishing debt repayment schedule in the next three months, the country now desperately needs those funds.
Last week, the parliamentary spokesman for Syriza had also said that the government would be unable to honour repayment to the IMF as its priority is to pay salaries, pensions and running costs.
"No country can repay its debts with only the money from its budget," Nikos Filis told Ant1 television.
