Seoul, South Korea — U.S. Defense Secretary Leon Panetta on Wednesday called North Korea a "serious threat" and told U.S. troops that the Pentagon will strengthen its presence in this region to guard against North Korean provocations.

Panetta met with top U.S. commanders here. In an opinion piece published by a Korean newspaper before he arrived in Seoul, he wrote that the U.S. will continue to cooperate with South Korea as a bulwark against potential North Korean aggression, "and stand prepared to defeat" the North if it should attack.

"It is important to send this signal because North Korea remains a serious threat," he wrote.

The U.S. has 28,500 troops stationed here - holdovers from the Korean War that ended in 1953 with a truce but no peace treaty.

Panetta's characterization of the North as a serious threat was a bit less forceful than earlier in the week, when he called the North Koreans "reckless." That was before U.S. and North Korean diplomats completed talks in Geneva about resuming full blown negotiations over the future of North Korea's nuclear program.