By Chris Lawrence

While North Korea continues to elevate threats against the United States and its allies, the Pentagon has not seen anything "out of the ordinary" around key missile sites, a defense official told CNN on Friday.

But the heightened rhetoric over nuclear attacks, so far unmatched by any actual military moves, has no foreseeable endgame, a second defense official said.

"This could go on for a while, and we could see variations of the rhetoric," the second official said.

North Korean leader Kim Jong Un has approved a plan to put rockets on standby to fire at U.S. targets, including the American mainland and military bases in the Pacific and in South Korea, state media reported Thursday.

South Korea's Yonhap News Agency on Friday cited South Korean military sources who said there was "brisk activity" by the North around missile sites.

But the second defense official told CNN that North Korea often carries out military exercises of its own around the same time that joint exercises are conducted by the United States and South Korean forces.

U.S. Joint Chiefs Chairman Martin Dempsey also suggested the activity the United States was observing was not out of the ordinary.

"There have been some artillery movements, but nothing other than what's consistent with their historic patterns and training," Dempsey said at a news conference on Thursday.

Still, the United States flew two stealth B-2 bombers near North Korea as part of military exercises on Thursday, viewed as demonstration of resolve to defend its allies and interests.

There were "intense discussions" inside the Pentagon and with the White House before the decision was made to fly the radar-evading planes over the Korean Peninsula, the second defense official said.

The B-2 requires a high level of approval before being deployed.

The official said the deliberations "went all the way up to the senior levels of the National Security Council" before Defense Secretary Chuck Hagel signed the order, the official said.

There was a "full range of options considered" before deciding to fly the B-2, according to the official.

"This was a well thought through decision. Everything about that flight was intentional: From the time of day, to the altitude, the planes that accompanied the B-2s and where it specifically flew."

The official said the B2 flew over Osan Air Base, about 50 miles south of the Demilitarized Zone and North Korean border.

"It's a unique, quite visible signal," the official said. "A message of reinforcement to our allies that the U.S. is open to bringing all assets to support them."

While clearly a message to North Korea that "we can bring force from anywhere and employ the nuclear arsenal from the U.S.," the official said the flight was weighted more toward reassuring South Korea and Japan.

"When they look up and see this, they know 'the U.S. has our backs.' And that is a very big deal in that part of the world, maybe more so than it would be in the US."

Without being specific, the official said the United States has a "range of options still out there, other things that we could do in the future."