Tensions between the two Koreas are being felt throughout East Asia since an international investigation blamed North Korea for sinking a South Korean warship.
In the first direct military response from the United States, the Pentagon announced Monday joint anti-submarine exercises with South Korea. Japan has cited the tensions as a reason for closer military cooperation with the U.S.
South Korea's President Lee Myung-bak gave a stern speech Monday at a war memorial in central Seoul. He said that his country would exercise its right to self-defense if its territory were violated by military force. He also pledged to bring North Korea before the U.N. Security Council and he suspended most trade between the two countries.
In Beijing, U.S. Secretary of State Hillary Clinton praised Lee's actions and agreed with South Korea's conclusion that the North had torpedoed the South Korean warship Cheonan, killing 46 sailors in March.
"The United States fully supports President Lee's responsible handling of the Cheonan incident, and the objective investigation that followed, which we and other international observers joined. The measures that President Lee announced in his speech are both prudent and entirely appropriate," Clinton said.
Pyongyang has denied involvement in the Cheonan incident.
The tensions come as U.S. forces are consolidating their bases and preparing to give wartime operational control of combined forces to South Korea in 2012. The debate on the wisdom of this move is now likely to heat up.
The Cheonan incident could lead to a more robust U.S. military posture in the region, said John Park, director of the Korea Working Group at the United States Institute of Peace in Washington, D.C.
"I think the portrayal of North Korea has been more of [weakness] and the concern of collapse being the greater threat. But with the Cheonan, it could be something of a game changer, where it's viewed as a potent conventional threat," Park said.
But other than reassurances and military drills, the U.S. has not said how it would beef up its military presence.
Thomas Fingar, a former deputy director of national intelligence and now a scholar at Stanford University, said he doesn't see any defensive holes that need plugging.
"I don't see a capacity deficit on the part of the U.S. in the region, the South Koreans, the Japanese. There's plenty of force capability out there now to deal with North Korea," Fingar said.
Meanwhile in Japan, residents of the island of Okinawa angrily protested Prime Minister Yukio Hatoyama's decision to move a U.S. Marine air base to another location on the island, despite his campaign pledge not to. Clinton said Hatoyama made the right choice.
"As a former politician, I know how hard Prime Minister Hatoyama's decision was, and I thank him for his courage and determination to fulfill his commitments," Clinton said.
China has responded coolly to the Cheonan investigation. Park, of the United States Institute of Peace, said Beijing sees security in being the only country with close ties to both Koreas.
"Picking one Korea over the other, to the Chinese, has always invited instability. And so right now, this notion that we need to get China onboard to support the South Korean-led efforts, I think it's very important to re-evaluate this in the broader context of what the Chinese are trying to pull off," Park said.
Fingar said that the U.S. force posture in Asia may eventually be scaled back, as it can project military force over greater distances.
"[In the] long run, we're probably going to draw down, because we've got long legs, we can get there in a hurry," Fingar said.
That will make U.S. forces less of a hostage to any potential adversaries nearby, he added.