Pentagon officials are closely watching four North Korean submarines that recently went to sea, prompting South Korean commanders to give their navy a green light to retaliate if fired upon.

Just days ahead of a controversial long-range rocket test and a major political conference in Pyongyang, North Korea reportedly quietly sent four of its diesel-electric submarines on deployment this week.

The North's large Sang-O-class and "midget" Yono-class submarines are not as stealthy as more-advanced American vessels, but they are still capable of eluding many modern sonar systems.

Pentagon spokeswoman Cmdr. Leslie Hull-Ryde says Defense Department officials "continue monitoring the situation very closely." U.S. officials "are consulting closely with our international partners on next steps," Hull-Ryde says.

But it's unclear what those "next steps" should be.

"South Korea has let it be known they've taken one too many punches," says Victor Cha, who directed Asian affairs for the National Security Council under George W. Bush. That punch-too-far came in March 2010, when a South Korean warship was allegedly sunk by a North Korean Yono-class sub along their disputed maritime border.

The North has denied it sunk the ship—which killed 46 sailors—and vowed to fight "all out war" if punished. Pyongyang never was reprimanded and the South never retaliated.

But that is unlikely to happen again.

"If the South is shot upon again by the North, there is no question in my mind that South Korea will respond," Cha says. "In the past, South Korean ship commanders were required to go up the chain of command for approval to shoot. After the 2010 incident, commanders were given a standing order to retaliate."

That means, Cha says, "there is a much clearer trigger this time for this to escalate."

An official at the South Korean embassy in Washington did not return a call seeking comment.

The U.S. military has remained largely quiet about the submarine deployment "because they're likely busy watching this situation," Cha says. "But the first U.S. response will be to remain in international waters and evade conflict."

U.S. officials have leaned on China, Pyongyang's lone true ally, to press the North about actions that could lead to a conflict.

"The Chinese have been trying to deliver that message to the North Koreans," Cha says. "They've been trying to tell them, 'Don't act like North Koreans and do something stupid.' So the Chinese are essentially stuck taking all the blame because they can't keep these people in line."

Beijing "is as frustrated as we are," Cha says, but will stop short of cutting off things like food aid to the North due to concerns about a refugee crisis spinning the region into a different flavor of turmoil.

Any hostile actions by the North's submarines would be a political bruise for President Obama, whose administration struck an apparent deal with the North under which the U.S. would send food assistance in return for Pyongyang halting nuclear testing.

Since that deal was announced last week, the North has begun fueling a long-range rocket that could one day be fitted with a nuclear warhead with enough range to target Hawaii or Alaska, and sent four submarines to sea.

Both actions seem coordinated around the Pyongyang political conference, at which new North Korean leader Kim Jong Un is expected to receive new titles and authorities.