U.S. bond supply, however, remained the focus of the week, with the $62 billion Treasury coupon auctions, beginning with Tuesday's $28 billion three-year note sale. That will be followed by the sale of $21 bln in 10-year notes on Wednesday, and $13 billion in 30-year bonds.
"Some of today's weakness is just prepping for the auction that's coming this week,'' said David Keeble, global head of interest rates strategy at Credit Agricole in New York.
"It's the long ones on auction, so you've got to put duration in the market and we're doing it now without much help from Federal Reserve purchases that we've had before.''
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Benchmark 10-year notes were last down 5/32 in price to yield 2.61 percent, from 2.59 percent late on Friday.
For the first time since April 2010, Spanish 10-year yields fell below those of U.S. Treasurys. Spanish yields hit historic lows of 2.58 percent.
Italian five-year yields were also below U.S. equivalents, highlighting the policy divergence between the European Central Bank, which has launched further stimulus, and the Fed, which has been reducing its asset purchases.
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The impending end of the Fed's quantitative easing has been helped by upbeat U.S. non-farm payrolls, which increased by 217,000 last month, returning employment to its pre-recession level.
U.S. five-year note prices were down 6/32 to yield 1.68 percent.
U.S. 30-year bonds fell 7/32 in price to yield 3.45 percent, from 3.43 percent late on Friday.
Investors are now looking to Tuesday's U.S. three-year note auction, which has seen mixed results in the past despite steady demand.
Barclays Capital in a research note said on a three-month moving average basis, domestic investment fund demand has increased to 26 percent since February, while foreign investor demand has declined by a similar proportion resulting in broker-dealers still absorbing 56 percent of the auctioned amount.
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The New York Federal Reserve, meanwhile, bought on Monday $0.97 billion in bonds with maturities ranging from February 15, 2036 through May 15, 2044. Offers totaled $4.559 billion.