Foreigners sold $74.6 billion in U.S. Treasury debt in the month, after purchases of $23.6 billion in March. April's outflow was the largest since the U.S. Treasury Department started recording Treasury debt transactions in January 1978.

Private offshore investors sold $59.1 billion in U.S. government bonds, while foreign official institutions, which include central banks, sold $12.3 billion.

U.S. economic data in April included a decent non-farm payrolls report for March, along with strong manufacturing as measured by the Institute for Supply Management. That prompted investors to sell Treasuries in April, as did an upswing in risk appetite, with buoyant global stocks and rebounding oil prices.

Yields on U.S. 10-year Treasury notes at the beginning of April were 1.7910 percent, and they hit a high of 1.9410 late in the month.