Yields on Greek bonds pushed to fresh highs on Monday and shares in Athens sank as investors continued to worry about the country's near-term ability to finance its debt. Some raised the specter of default even if international aid arrived soon. Analysts said traders shifted out of Greek bonds amid worries about the country's ability to raise more than 11 billion euros, or $14.8 billion, in May, especially in light of the delay of a crucial meeting that was to be held Monday between European, Greek and International Monetary Fund officials. Vassilis Papadimitriou, a spokesman for the Greek government, said the meeting, postponed because of travel chaos in Europe, would go ahead Wednesday provided that European Union and central bank officials were able to get to Athens. Airports across Europe remained closed Monday because of the volcanic eruption on Wednesday in Iceland, which has spread potentially hazardous ash across the Continent. Bill Murray, a Washington-based spokesman for the IMF, confirmed on Monday that most members of the IMF team had arrived in Athens to discuss 'policies that could be the basis for a fund financing program.' He declined to comment on the duration of the mission. Even with those talks expected to move ahead, investors were worrying about the country's financing needs -- in coming months and years. 'The market wants it clearly pinned down that the money is there and ready to go,' said David Schnautz, a strategist at Commerzbank in Frankfurt. The Greek government's financial position 'looks safe for April, but there is uncertainty about May,' he said. But it is not clear what debt maturities the agency will use to raise those funds, Schnautz said, adding that in recent weeks there had been 'an erosion of the investor base' in Greek bonds. Another factor that appears to have scared potential buyers was a recent suggestion by Greece that it might issue dollar-denominated bonds -- something that some investors associate more with emerging-markets issuers than established euro zone borrowers. In addition, there is still a risk of a veto on the package of up to 30 billion euros in aid promised this month by lawmakers from some euro zone countries. The Greek government plans to auction 1.5 billion euros of three-month Treasury bills Tuesday, providing some idea of the level of return that investors expect for holding short-term Greek debt. Further ahead, some analysts are starting to foresee default as likely even with the bailout, asserting that Greece does not have the economic flexibility to emerge from the downturn. 'An eventual default by Greece seems probable, and rescue packages by the IMF and the EU may only buy time but not alter the ultimate outcome,' Stephen Jen, managing director of macroeconomics at BlueGold Capital in London, said in a recent research note. He drew parallels with the case of Argentina, which defaulted in 2001. Both examples involved 'weak fiscal management, an uncompetitive industrial structure, cutting spending in a weak economy, open capital accounts and an inappropriate currency peg prior to the crisis,' Jen said. He added Greece's fiscal standing was 'actually substantially weaker than Argentina's' on the eve of that country's default in 2001. The yield on benchmark 10-year Greek government bonds closed in Europe at 7.63 percent -- the highest since Greece joined the euro. That widened the spread, or difference, with equivalent German bonds to 4.55 percentage points. The Athens composite share index closed down 2.8 percent, pushing it down almost 12 percent for the year to date. Investors also showed jitters Monday about Portugal's ability to finance its debt load. Portuguese 10-year government bonds yields also climbed, to 4.56 percent, a gain of 12 basis points and their highest level since March 2009. Though facing less severe constraints than Athens, many investors say they think that Lisbon could be the next in the euro zone to seek international aid, given its weak economic fundamentals and high budget deficit. Separately, the European Central Bank, in a report Monday on the lessons learned from the financial crisis, recommended strengthening the supervision of over-the-counter derivative markets. 'The soundness, resilience and transparency of OTC derivatives markets should be enhanced,' the report said, stressing the need for progress toward establishing central counterparties to smooth the process of clearing. The European Commission issued a paper on planned action to improve the resilience of these derivatives markets in October and legislative proposals are expected soon. The central bank also warned investors against excessive reliance on ratings agencies, which the bank characterized as having been guilty of 'changing the rating of a critical counterparty's creditworthiness too late.' 