S Korea finds holes in North's border fence
A spokesman said it was highly possible that North Korean spies or commandos had infiltrated into the south.
Roadblocks have been set up and officials said reservists might be called up to search for any intruders.
The alert coincides with a visit by US Secretary of State Colin Powell, who is on the final leg of an East Asian tour.
Powell has been trying to revive stalled six-way talks on North Korea's nuclear weapons plans.
The two holes, about 40cm (16 inches) wide, were found in the parallel fences that mark the southern boundary of the 4km-wide demilitarised zone that separates the two Koreas.
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