The U.S. Treasury Department announced Monday that the federal government expected to borrow 644 billion U.S. dollars from the market to fund its operation in the first half of this year, fresh evidence of the budgetary pressure facing the world's largest economy.

"During the January-March 2012 quarter, Treasury expects to issue 444 billion dollars in net marketable debt, assuming an end- of-March cash balance of 30 billion dollars. This borrowing estimate is 97 billion dollars lower than announced in October 2011," the Treasury said in a statement.

The Treasury forecast that 200 billion dollars will be issued in net marketable debt during the April-June quarter this year, assuming an end-of-June cash balance of 90 billion dollars, noted the statement.

U.S. federal budget deficit hit almost 1.3 trillion dollars in fiscal year 2011 ending in September 2011. The red ink of the federal government budget reached the historic high level of 1.41 trillion dollars in fiscal year 2009 to fight the financial crisis.

"During the October-December 2011 quarter, Treasury issued 310 billion dollars in net marketable debt, and ended the quarter with a cash balance of 86 billion dollars," according to the statement.

Due to the financial crisis and shrinking revenue, borrowings of the U.S. federal government surged over the past three years and the trend will continue this year.

Experts held that the U.S. economy could outperform most advanced economies in the euro area, but the unsustainable federal budgetary trajectory and still weak labor market caused uncertainties for business investment and household consumption.

The U.S. economy was facing a string of challenges, including the slowdown in growth overseas, particularly in Europe, and the threat of further fiscal contraction at all levels of the U.S. government, Janice Eberly, Assistant Treasury Secretary for Economic Policy, said in a separate statement released Monday.

"At this critical juncture, we need to remain consistent in our support for the recovery," stressed Eberly.