LISBON, Portugal — Portugal needs to raise up to €20 billion ($26.5 billion) on international markets in 2011, the national debt agency said Wednesday, in what will be a key test of the country's ability to calm jittery investors.

Portugal is one of the frailest members of the 16-nation eurozone. Its high debt burden and low growth in recent years have fueled speculation it may join Greece and Ireland in needing a financial bailout.

The Institute for Treasury and Public Credit Management said it intends to issue bonds worth €18-20 billion to meet Portugal's financing requirements next year. It provided no dates for the auctions.

The institute said it also plans to issue shorter-term Treasury bills next year though the sales will depend on market conditions. The first sale, of €500 million in three-month bills, is scheduled for Jan. 5.

So far, the country has been able to raise the money it needs on international markets but the worry is that the higher rates demanded by investors could become unsustainable — as happened with Greece and Ireland.

The yield on Portuguese 10-year bonds has risen in recent days, hitting 6.7 percent Wednesday. By comparison, the yield on benchmark German bonds stands at around 3 percent.

Portugal has to raise money because its budget deficit is too high — though its budget deficit of 9.6 percent last year was far below Greece's 15.4 percent, it was still the fourth-highest in the eurozone.

The government says Portugal's deficit will fall to 7.3 percent this year and to 4.6 percent — one of the lowest in the eurozone — in 2011, and insists that it doesn't need or want a financial rescue.

Though the austerity measures adopted by the government to restore the country's fiscal health are reducing the deficit, they are likely to stunt economic growth next year.