Spanish bond yields have risen above 7 percent, the level considered by analysts as unsustainable in the long-term.
The rise comes ahead of a meeting of euro zone finance ministers on Monday, where the delegates are expected to announce more details of the Spanish bank bailout, the BBC reported.
Italian bond yields also rose to 6.1 percent on Monday, leaving analysts to once again question the ability of the euro zone's most indebted members to service their crippling public deficits.
The Spanish and Italian numbers were in contrast to Monday's German short-term bond auction.
Rates there fell to -0.03 percent, meaning investors are effectively paying the German government to lend money.
Despite the negative yield -- the second time this has occurred -- the auction was oversubscribed as nervous investors continued to flock to German debt as a safe haven.
Elsewhere, euro zone finance ministers warned not to expect too much from Monday's two-day summit, which is intended to flesh out details of the previous meeting on June 29.