Issuers are returning to the U.S. corporate bond market this week, in the strongest start in more than a month, as Spain's request for a bank bailout reduced concern that the European debt crisis is spreading.

AT&T Inc., the largest U.S. phone company, and Louisville, Colorado-based Zayo Group LLC are leading sales of at least $5.7 billion, the best opening for a week since May 7, according to data compiled by Bloomberg. Sales totaled $16.3 billion last week, a 7 percent increase from the prior week, though below this year's average of $28.3 billion for the fourth straight week.

Issuance is picking up after Spain asked euro-region governments over the weekend for as much as 100 billion euros ($125 billion) in bailout funding to help shore up its banking system. The development has calmed some concern that Spain's strains could exacerbate Europe's debt crisis and weigh on corporate balance sheets worldwide. The reprieve may be short-lived, given speculation that aid for Spain's banks may not be enough to stem the European fiscal turmoil and as Greece holds elections on June 17.