By Min Zeng 

Yields on benchmark 10-year U.S. government debt fell to a two-month low on Monday as a disappointing manufacturing release cast doubt on a rise in interest rate by the Federal Reserve in September.

The monthly manufacturing index fell to 52.7 last month from 53.5. It followed Friday's release showing 0.2% rise in the employment-cost index Friday, the smallest in three decades and pointing to sluggish wage inflation.

The Fed's ultra-loose monetary policy following the 2008 financial crisis has sent prices of many stocks and bonds to elevated levels, raising concerns about whether financial assets could hold up once the Fed starts a tightening campaign for the first time in nearly a decade.

Fed officials have said the timing for the first rate increase since 2006 depends on how the economy performs. While the U.S. economy picked up speed in the second quarter, Fed officials and investors are grappling with sluggish global economic growth, subdued inflation and highly accommodative monetary policy around the globe. Over the past month, falling commodities and China's stock-market turmoil have heightened concerns over the slowdown of the world's second-largest economy and its potential knock-on effect to the rest of the world.

The yield on the benchmark 10-year Treasury note fell to as low as 2.168% on the manufacturing data, the lowest intraday level since June 1. Yields fall as prices rise.

Monday's report "is just another data point in this two steps forward one step back economy," said Larry Milstein, head of government and agency trading at R.W. Pressprich & Co. in New York. "On its own it does not move the timing of a September hike to December, but combined with Friday's weak inflation report and if we get a soft employment report on Friday, we could see the Fed delay a rate hike."

In recent trading, the yield was 2.173%, compared with 2.207% on Friday, according to Tradeweb.

The yield has dropped from this year's intraday peak of 2.5% set in June. Falling commodities have reduced inflation expectations. Inflation is a main threat to the value of longer-dated bonds as it chips away bonds' fixed return over time. Concerns over China's economy have also boosted demand for ultrasafe U.S. government debt. Investors who expect the Fed to wait until December or later to tighten also took advantage of recent higher yields to buy.

Monday's data underscore the challenges for the Fed to dial back crisis-era monetary policy amid sluggish global economic growth, subdued inflation and highly accommodative monetary policy around the globe.

Money managers and traders say volatility in the financial markets would rise leading into the September policy meeting as economic releases will drive the Fed's rate policy outlook.

"We will go number to number from now on," said Thomas Roth, executive director in the U.S. government bond trading group at Mitsubishi UFJ Securities (USA) Inc. in New York.

This week's key data point will be the nonfarm jobs report due on Friday. The U.S. economy is forecast to have added 215,000 new jobs in July following a net gain of 223,000 in June, according to the median forecast from economists polled by The Wall Street Journal. The average hourly wages were forecast to rise by 0.3% in July after being flat in June.

Many investors say a solid jobs report would bolster the case for the Fed to raise the fed-funds rate in September. Money managers and traders have migrated cash out of shorter-dated Treasury notes and into longer-dated bonds to position for the Fed's upcoming tightening cycle.

In the past three tightening cycles, the yield on the two-year Treasury note rose at a faster pace than the yield on the 10-year Treasury note. That is because shorter-dated bond yields are pinned by the Fed's short-term policy rate so they are more vulnerable to a shift in the Fed's policy.

Many money managers expect the 10-year yield to rise modestly during the balance of this year, seeing it as a normalization of the bond market in response to the Fed's shift away from ultraloose monetary policy. But they expect the Fed's gradual approach in tightening and contained inflation to keep a lid on a rise in the 10-year yield.

U.S. bond yields are higher compared with their counterparts in many other developed countries, including Germany, Japan, the U.K. and France. A stronger dollar boosts foreign investors' investments in U.S. financial assets.

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