Most strategists expect the Fed to refrain from a third round of quantitative easing, known as QE3, though the central bank is expected to emphasize its commitment to maintaining easy policy and could signal that further stimulus steps are coming

Investors are also wary ahead of a European Central Bank meeting on Thursday as they don't know what will come out of it. Late last week, ECB president Mario Draghi said he would do "whatever it takes to preserve the euro."

"The market is positioned for uncertainty again ahead of the Fed and the ECB," said a fixed-income fund manager at a Japanese asset management firm.

US Treasury Secretary Timothy Geithner called on European leaders to do more to solve the region's debt crisis, including lowering interest rates for countries that are undertaking painful reforms.

Interviewed on Bloomberg Television in Los Angeles on Tuesday, Geithner also said there was more the US Congress could do to bolster American growth.

The yield on the 10-year notes edged up slightly to 1.479 percent from 1.476 percent in late US trade on Tuesday.

The yield on 30-year bonds stood at 2.557 percent, slightly down from 2.560 percent in Thursday's late US trade.

Underpinning demand for safe-haven fixed-income assets, Chinese economic data highlighted slowing momentum there. China's official factory purchasing managers' index fell to 50.1 in July, below expectations and down from 50.2 in June. It was the lowest reading since November.