(Bloomberg) -- Treasuries rose as investors sought the safety of sovereign debt after a series of bombings in Brussels killed at least 31 and injured more than 180.

U.S. securities advanced with German bunds and U.K. gilts as three blasts in an airport and a subway station put Belgium on the highest terror-alert level. The difference between yields on two- and 30-year Treasuries, known as the yield curve, narrowed. U.S. stocks fluctuated.

"It raises more defensive positioning in risk assets, and puts money into Treasuries," said John Briggs, head of strategy for the Americas in Stamford, Conn., at RBS Securities, one of the 22 primary dealers that trade with the Federal Reserve.

The gains add to a rally that's pushed Treasuries to a 1.7% return this year, according to Bloomberg bond index data. Yet similar events in Europe, such as the 2004 train bombings in Madrid and the 2005 London Underground bombings, caused U.S. debt to surge before the moves faded later in the trading session. The most recent major attack in Paris in November had a negligible effect on Treasuries since it took place in late U.S. hours on a Friday.


"The market has become slightly more skeptical of pushing yields down too much," said Ian Lyngen, a government-bond strategist at CRT Capital Group in Stamford, Conn. "All things being equal, we think this will be a relatively short-lived flight to quality."

Benchmark U.S. 10-year note yields fell one basis point, or 0.01 percentage point, to 1.91% as of 1:18 p.m. New York Time, according to Bloomberg Bond Trader data, after falling as low as 1.88%. The 1.625% security due in February 2026 rose 3/32, or 94 cents per $1,000 face amount, to 97 15/32.

The British pound declined the most among major currencies amid speculation that the explosions may boost the chances of a British exit from the European Union, or Brexit.

"A greater risk of a Brexit means more uncertainty in general, in the world and the U.K.," Briggs of RBS said. "That makes U.S. government debt more attractive."

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