The European Central Bank's announcement Thursday that is intensifying its easing program gave impetus for a further drop in European bond yields, which have reached historic lows.

European government bond yields, even in troubled economies such as Italy and Spain, stand below U.S. Treasury yields, making Treasurys an attractive investment.

"Whether it's Japanese banks or sovereign-wealth funds, there's demand to put global capital into assets," Jurrien Timmer, director of global macro at Fidelity Investments, told The Wall Street Journal. "The U.S., among the major high-quality bond markets, is by far the cheapest there is."

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U.S. 10-year Treasurys yielded 2.43 percent Monday morning, compared with 0.93 percent for similar German bonds, 1.26 percent for French bonds, 2.05 percent for Spanish bonds and 2.27 percent for Italian bonds.

The drop in European government-bond yields makes Treasurys "look like a fire sale," Phil Camporeale, a member of J.P. Morgan Asset Management's Global Multi Asset Group, told The Journal.

Also helping Treasurys is Friday's news of a weaker-than-expected 142,000 gain in non-farm payrolls during August.

"This is the perfect excuse for the Fed not to move early" in raising interest rates, William Larkin, a portfolio manager at Cabot Money Management, told Bloomberg.

"It's a little on the weak side. It will keep the bond market in play for a while. We'll probably drop below the 2.4 percent range" for the 10-year Treasury yield.

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