WASHINGTON — (CNN) The US and South Korean militaries started a massive joint annual exercise Monday, a drill that has drawn North Korean threats of nuclear retaliation.

North Korea will “turn the stronghold of provocation into a heap of ashes through Korean-style pre-emptive nuclear strike” if the US and South Korea “show the slightest sign of aggression” during the drill, a spokesman for North Korea’s military was quoted as saying by the country’s state media.

North Korean forces are “ready to mount a pre-emptive retaliatory strike at all enemy attack groups involved in Ulchi Freedom Guardian,” he said, referring to the exercise by its official name.

The exercise takes place almost exactly one year after North Korean troops shelled South Korean territory and only days after South Korea welcomed the highest-ranking North Korean official defector in decades.

That defection, of Pyongyang’s deputy ambassador to the UK, caused North Korea to issue a statement calling the diplomat a criminal, with South Korean officials expressing concerns that North Korea might dispatch assassination squads to eliminate potential defectors abroad.

The annual drill will include 25,000 US troops, the bulk of which are already stationed in Korea, according to a statement by US Forces Korea.

The purpose of the exercise is to “enhance Alliance readiness, protect the region and maintain stability on the Korean peninsula,” the statement said.

South Korean President Park Geun-Hye responded to Pyongyang’s threats by saying, “The North Korean regime has been continuously suppressing its people by its reign of terror while ignoring the livelihood of its people.”

She added that the South would “prepare” for any possible North Korean provocations, in comments at a National Security Council meeting on Monday.

North Korea’s threats come amid new concerns over the increased sophistication of that country’s nuclear and missile programs.

“North Korea’s nuclear and missile threats are direct and realistic,” Park added.

A US official told CNN earlier this month that the regime’s aggressive testing of medium- and long-range missiles — as well as its nuclear testing — makes North Korea now a “practical” threat and no longer a “theoretical” threat.

“We obviously watch the situation very closely,” Pentagon spokesman Peter Cook told reporters Monday.

“It is an annual exercise. We have seen the rhetoric from North Korea, we have seen that rhetoric in the past, and of course we would urge the North Koreans to do what they can to ease tensions on the Korean Peninsula,” Cook said. “That is certainly something we would welcome.”

Victor Cha, the Korea chair at the Center for Strategic and International Studies in Washington, told CNN that North Korean leader Kim Jong Un is engaging in “much more aggressive” and “much more provocative” behavior than his predecessor and father, Kim Jong Il.

“We always see North Korea threatening during US-(South Korea) annual military exercises” he said, adding that North Korea does “often provoke around the time of these exercises.”

But Cha noted a few differences this time around, including the fallout from “a very embarrassing high-level defection” and North Korea’s desire to try to gain leverage amid the upcoming US presidential elections.

“North Korea has a propensity to try and capture everyone’s attention,” Cha said.

The US military has also been beefing up its presence in the region, jointly deploying three strategic bombers, the B-52 Stratofortress, B-1B Lancer and B-2 Spirit, to the region for the first time.

When asked if the US was concerned about the increase is hostile rhetoric out of Pyongyang, US State Department spokesman Mark Toner answered, “There is always concern, but I don’t think that is going to keep us from moving forward.”