ATHENS, Greece — The International Monetary Fund on Thursday approved $36.56 billion in funding for crisis-hit Greece over the next four years, while Standard and Poor's warned that the country's new bonds remained vulnerable to default despite this month's massive debt writedown.

The IMF's executive board granted the immediate release of $2.15 billion of these funds as part of the country's second bailout, a statement said.

Greece will receive a total $225.8 billion in rescue loans from its eurozone partners and the IMF to keep it afloat until 2016, as dizzily high borrowing rates have blocked its ability to raise money on the international bond markets.

IMF Managing Director Christine Lagarde warned that risks to Greece's austerity and reform program still "remain exceptionally high, and there is no room for slippages." She said new pain lies ahead for Greeks, despite the tough measures implemented over the past two years.

"Full and timely implementation of the planned adjustment — alongside broad-based public support and support from Greece's European partners — will be critical to success," she said in a statement.

"Significant further fiscal adjustment is necessary to put debt on a sustainable downward trajectory," Lagarde said, adding that politically difficult additional spending cuts worth about 5.5 percent of GDP — about $14.3 billion — lie ahead in 2013 and 2014.

Chief IMF inspector for Greece Poul Thomsen said Athens should focus on spending cuts, improving tax administration and fighting tax evasion, rather than further hiking taxes.

He added that Greece could regain market access before 2020, although it would have to accept relatively high interest rates at first.

Without the bailout, Greece would have been forced into a messy default of a $18.9 billion bond repayment due Tuesday, a move that could have sent shock waves throughout the global financial system and further destabilized the group of 17 countries that use the euro.

The new bailout cash was approved after Greece secured a massive debt-reduction deal with banks and other private bond holders, swapping old government bonds for new ones that have better repayment terms.

Ratings agency Standard and Poor's assigned a CCC score — or still vulnerable to default — and said Greece's sovereign rating would remain in selective default until the exchange was completed next month.

Following the exchange, the agency said it would "likely consider Greece's selective default to be cured" and assign the country a CCC sovereign rating.