Mounting fears of a Greek debt default have sent the country's borrowing costs in the markets surging higher.

The latest jitters were stoked by a report Thursday in the Financial Times that Greece made an "informal approach" to the International Monetary Fund to have its bailout repayments delayed.

Citing unnamed officials from both sides, the newspaper said Athens was persuaded not to make a request. For investors, the report was unsettling and the yield on Greece's 10-year bonds surged a whole percentage point to just under 13 percent.

Greece owes the IMF around 1 billion euros ($1.06 billion) in repayments next month. Many in the markets think the Greek government will struggle to make those payments if it doesn't agree an economic reform package with European creditors soon.