By Min Zeng 
 

U.S. Treasury bonds pulled back Wednesday as investors took some chips off the table after a rally during a prior session.

Looming new debt sales weigh down bond prices. A $35 billion sale of five-year notes is due at 1 p.m. EDT Wednesday, followed by a $29 billion sale of seven-year notes on Thursday.

In recent trading, the yield on the benchmark 10-year Treasury note was 2.151%, compared with 2.135% on Tuesday, according to Tradeweb.

Bond prices fall as their yields rise. The yield was 2.173% at the end of 2014.

Investors piled into haven Treasury bonds Tuesday, sending the 10-year note's yield to a three-week low, driven by renewed worries over Greece and a selloff in U.S. stocks.

Greece's government and its international creditors have been stuck in negotiations over terms for funding. The deadline for cash-strapped Greece to repay loans to the International Monetary Fund is early next month.

A primary concern is that Greece could default on its debt, which could raise the risk of the country exiting the eurozone, a scenario that would cause broader market turmoil.

Contagion from Greece into markets of other weaker economies in the eurozone has been contained over the past few months. Some investors still expect a deal to be reached to avoid a default. Meanwhile, the European Central Bank has been buying bonds since March to support the economy and investors say this monetary backdrop would help contain the risk of spillover.

Traders expect the uncertainty to fuel more market swings. Wednesday, the yield on the 10-year Greek government bond fell after soaring on Tuesday. The yield was recently down about 0.15 percentage point at 11.762%.

Spain's government bonds also rebounded after Tuesday's selloff. The 10-year government bond yield in Spain fell by 0.02 percentage point to 1.857%.

Besides Greece, a main focus for bond investors is when the Federal Reserve is going to start raising short-term interest rates and what will be the pace of a new tightening cycle.

A major shift by the central bank into a tightening mode for the first time since 2006 would send prices of outstanding bonds lower as higher policy rates make newly sold bonds more attractive to buy.

Mixed economic releases over the past few weeks have raised the question over how robustly the U.S. economy is rebounding. Many investors expect the Fed to wait until late this year to raise interest rates.

"The economy continues to muddle along," which means the Fed would not be in a hurry, said Ford O'Neil, a portfolio manager at Fidelity Investments, which manages $2.1 trillion in global assets. A slow path of tightening will prevent Treasury bond yields from rising significantly, he said.

Some traders caution that the Fed may raise rates sooner than many investors expect, a case that would rattle the bond market and send yields climbing.

The 10-year Treasury yield touched 2.366% earlier in May amid a weeks-long rout in government bonds from both sides of the Atlantic. The selloff reflects investors' concerns that the bonds' valuations are getting stretched after a strong run-up since the start of 2014. The 10-year German government bond yield fell to a record low near zero last month, driven by the ECB's bond-buying program.

Write to Min Zeng at min.zeng@wsj.com