The US has about 28,500 troops on the Korean peninsula, including more than 16,000 soldiers guarding the "demilitarized zone" between North and South Korea. On Thursday, the US and South Korea raised the threat level there to its highest point in 2-1/2 years, in response to Pyongyang's actions.
But there are few military options to counter North Korea's move, and analysts say most of them would seem aggressive and only ratchet up the tension.
"As North Korea escalates day by day, you don't want to be provocative," says Nicholas Szechenyi, a fellow at the Center for Strategic and International Studies, a think tank in Washington. The US military should sit this one out because this is a time for diplomacy, not a show of force, he says.
"I don't think that anybody in the [Obama] administration thinks there is a crisis," Mr. Gates told reporters aboard his military jet early Friday morning, still Thursday night in Washington.
"What we do have, though, are two new developments that are very provocative, that are aggressive, accompanied by very aggressive rhetoric," he said. "And I think it brings home the reality of the challenge that North Korea poses to the region and to the international community."
The missile tests, including a new one Friday, no doubt will feature prominently at the three-day Shangri-La Dialogue, which brings together Japan, South Korea, and China, among other nations. In seeking to reassure America's Asian allies, Gates will have to navigate a fine line between diplomacy and military muscle-flexing there, experts say.
"I think the military role in this is to reassure the South Koreans that we're engaged," says Chris Hellman, military policy fellow at the Center for Arms Control and Non-Proliferation, a policy group in Washington. The best way to counter North Korea's bellicosity, he says, is to signal that American support of its South Korean ally is unwavering.
This week, the Defense Security Cooperation Agency notified Congress that it planned to provide a $250 million upgrade to 35 South Korean F-16 fighters that would allow the deployment of Joint Direct Attack Munitions and Advanced Medium Range Air-to-Air Missiles as part of a package of upgrades, training, and equipment.
A separate contract for about $170 million for 84 missiles of different kinds will "enhance" South Korea's defensive capabilities, according to the agency.
With so many troops in the region, the US military could conduct new naval or even ground exercises as a show of power. But such exercises – typically thinly veiled chest-beating aimed at sending a message – could be seen as overly aggressive. Also, they would take months to plan.
Expanding a naval embargo would also be interpreted as aggression.
Such "moves on the chessboard" would be overly provocative at a time when the situation between the US, its allies, and North Korea is precarious, Mr. Hellman says.
North Korea has kept the Pentagon up at night for years, in part because of its mercurial leader, Kim Jong Il. Mr. Kim's focus is to ensure his regime stays intact, even as the country confronts an economic crisis that could make him all the more desperate, said Gen. Walter "Skip" Sharp, the top US commander in the Republic of Korea, during a House panel hearing in March.
The military has a "contingency plan" for any security scenario, and this region is no different. Should security in the region unravel completely and North Korea attack South Korea, the US has a secret war plan called "5027" that would provide the required American troops to South Korea to ward off the attack.
"I'm absolutely confident if North Korea were to attack today, we, the Republic of Korea-US alliance, would be victorious and we would be able to execute our war plan, 5027," General Sharp said.
For now, the US military only has plans to bolster South Korea's military capabilities and leave it to American and international diplomats to sort out.
At any rate, it's important for the US to be deliberate in its response to an act of aggression that is likely to be more bark than bite, say analysts.
"It's all smoke; these guys have no cards to play," Mr. Hellman says.
• Associated Press material was used in this report.