SEOUL, South Korea — South Korea is investigating signs North Korean agents might have infiltrated the world’s most heavily fortified border on the day America’s top diplomat is visiting the nation.

South Korean border guards reportedly found a hole in the wire fence that forms the southern boundary of the demilitarized zone (DMZ) separating the two Koreas.

The hole, which measured about 40 cm by 30 cm (16 inches by 12 inches), was found near Yeoncheon, a border town 60 kilometers (37 miles) north of Seoul.

Alerts in the DMZ are taken seriously in the South and the military has imposed the highest level of vigilance, tightening roadblocks and setting up checkpoints in the area on Tuesday.

Under this heightened alert, military units can move troops for patrol and combat readiness, with soldiers joining police at checkpoints.

It is considered unlikely that civilians could have made their way past guards and land mines in the heavily patrolled DMZ.

The two Koreas have been on a hair- trigger for 50 years, ever since the last shot was fired in the Korean War and an uneasy truce came into force.

Almost 2 million troops face each other on either side of the 248-km (151-mile) DMZ, ready to go to war at a moment’s notice.

The security alert comes as U.S. Secretary of State Colin Powell visits South Korea on the last leg of a three-nation Asian tour to revive stalled talks on North Korea’s nuclear weapons programs.

The United States has been spearheading the six-nation talks — also involving the two Koreas, China, Japan and Russia — in a bid to get Pyongyang to dismantle its programs.

China was scheduled to host the fourth round of talks by the end of September but they were indefinitely postponed, with North Korea and the United States at loggerheads and blaming each other for the impasse.

‘Impossible to enter talks’

Washington is pressing Pyongyang to completely halt its nuclear activities, but North Korean officials have accused the United States of harboring “hostile policies” against them and are demanding economic concessions and security guarantees.

The two sides also disagree over the nature of Pyongyang’s nuclear program, the sequence of dismantling it, and the concessions Pyongyang could get from the United States and other countries in exchange for such disarmament.

In Seoul on Tuesday, Powell reiterated Washington’s position that it does not intend to attack North Korea and that it was time “to bring this matter to a conclusion.”

“We don’t intend to attack North Korea, we don’t have any hostile intent notwithstanding their claims,” Powell said at a news conference with Ban Ki-Moon, South Korea’s foreign minister.

But North Korea on Tuesday said it was impossible to enter nuclear talks at this time and accused the United States of playing politics with the talks ahead of next week’s elections.

“The nuclear issue on the Korean Peninsula has become a topic of the debate during the election campaign, and there is a hot argument about who is to blame for the delay of the six-party talks with the presidential election fast approaching,” North Korea’s official KCNA news agency said.

“Under this situation the Bush administration is employing a sleight of hand to mislead the public opinion at home and abroad and garner support from more electors.”

‘Terrorist state’

Powell has met with Japanese, Chinese and South Korean officials in recent days in a trip widely seen as an attempt to show resolve on one of the Bush administration’s most difficult foreign policy issues.

U.S. President George W. Bush has linked North Korea with Iran and Iraq in an “axis of evil.”

Democratic presidential nominee John Kerry says the Bush administration has mishandled the North Korea problem and should have embraced the Clinton-era policy of direct talks with the country.

But Bush administration officials believe it is better to deal with Pyongyang multilaterally, involving North Korea’s neighbors as they are at a greater risk from a nuclear war than the United States.

“North Koreans desperately want to make this a U.S.-North Korean problem to see what else they can ask us for, to pay them, to reward them for their misbehavior,” Powell told CNN’s Mike Chinoy in an interview on Monday.

“We have chosen not to do that, not to get caught in their trap again.”