SEOUL: North Korea on Tuesday called the ongoing South Korean-United States military drills a plot to invade the country, and threatened to take "responsible" military steps in response.

The North's warning came a day after South Korean and American forces kicked off their annual computer-simulated command post training and field exercises that are expected to last 11 days. This year's drills are to involve 48 field exercises, doubling last year's number.

In a statement carried by state media, the North's Defense Ministry said it "strongly denounces the reckless military drills of the US and [South Korea] for getting more undisguised in their military threat to a sovereign state and attempt for invading it."

An unidentified ministry spokesman said Pyongyang's armed forces would "continue to watch the adventurist acts of the enemies and conduct responsible military activities to strongly control the unstable security environment on the Korean Peninsula."

The spokesman didn't say what measures North Korea would take, but observers say the country is likely to conduct missile tests or other steps to bolster its war capability.

South Korea's Defense Ministry said later on Tuesday that the drills were regular, defensive training. A ministry statement said Seoul would issue an overwhelming response if Pyongyang launches direct provocations against the country during the exercises.

North Korea views its rivals' drills as invasion rehearsals, though South Korean and US officials have repeatedly said they have no intentions of attacking the North. Pyongyang has previously reacted to such exercises by launching a barrage of missiles into the sea.

South Korea's Joint Chiefs of Staff said last week this year's drills with the US were designed to neutralize North Korean nuclear threats and would involve live-firing, bombing, air assault and missile interception drills.

Concerns about North Korea's nuclear program have grown in the past two years, as it has test-launched missiles at a record pace and openly threatened to use nuclear weapons preemptively. Washington and Seoul have expanded their military exercises and increased the deployment of powerful US military assets like aircraft carriers and nuclear-capable bombers in response.

This year, North Korea conducted six rounds of missile tests and artillery firing drills. Its leader Kim Jong Un has declared that his country won't seek reconciliation with the South and vowed to scrap Pyongyang's long-running goal of peaceful unification with the South.

Experts say North Korea could believe that a bigger weapons arsenal would provide it with greater leverage in future diplomacy with the US. Pyongyang, they add, is desperate to win international recognition as a nuclear state, a status that it would think helps it win relief from US-led economic sanctions.

North Korea is expected to further dial up tensions with more missile tests and warlike rhetoric this year as the US and South Korea head into major elections. The North may stage limited provocative acts near its border with the South this year, experts say.