U.S. bond prices extended early gains after two economic readings showed unexpected weakness.
U.S. industrial production declined 0.2% in May, compared to the 0.2 percent increase expected. A separate report showed factory activity in New York state contracted in June, a sign that manufacturers are held back by a strong dollar and cutbacks in investment by oil and gas drillers.
U.S. Treasurys started the week on a firm note on Monday as jitters about Greece boosted the appeal of safe-haven bonds, pushing yields broadly lower.
Talks between cash-strapped Greece and its creditors for much-needed aid in return for reforms collapsed at the weekend, taking Athens a step closer to a debt default that could end with its exit from the single-currency zone.