Washington, Aug 2 (IANS): A "highly unusual and unprecedented levels" of North Korean submarine activity and evidence of an "ejection test" have been detected by the US military, the media reported on Wednesday.

The US has observed the activities in the days following Pyongyang's second intercontinental ballistic missile launch in a month, CNN quoted a a defence official as saying.

An ejection test examines a missile's "cold-launch system", which uses high pressure steam to propel a missile out of the launch canister into the air before its engines ignite.

That helps prevent flames and heat from the engine from damaging either the submarine or any nearby equipment used to launch the missile.

Sunday's test is the third time in July that North Korea has conducted a trial of the missile component that is critical to developing submarine launch capabilities, said the official.

US officials also said a North Korean Sang-O submarine was operating in the Yellow Sea and the length of its deployment was notable.

Two Romeo submarines were detected in the waters off Japan -- each one operating in the area for about a week.

Coupled with reports of increased submarine activity, news of another ejection test comes amid concerns over the Communist nation's launch of an intercontinental ballistic missile that has the "range to hit US cities".

Experts believe if Friday's test had been fired on a flatter, standard trajectory, it could have threatened cities like Los Angeles, Denver and Chicago.

President Donald Trump said his administration will take care of North Korea but offered no specifics about what he plans to do.

"We will handle North Korea. We are gonna be able to handle them. It will be handled. We handle everything," Trump said after a reporter asked him about his strategy.

Asked if the US would strike first, Trump's press secretary Sarah Huckabee Sanders said "all options are on the table", adding that Trump would not "broadcast" his decisions.

North Korea's submarine fleet is believed to encompass about 70 subs, though the majority are quite old and likely cannot fire missiles.

When taken together, these developments are concerning because North Korea said it was trying to develop a missile capable of delivering a nuclear warhead to the US.

CNN reported earlier in July that North Korea had sent a diesel-powered Romeo Class sub on an unprecedented patrol and had been outfitting its Gorae Sub with a possible missile launch demonstration tube.

Two US defence officials told CNN at the time that the Romeo-class submarine was engaged in "unusual deployment activity" in the waters off the coast of Japan and was patrolling farther that it has ever gone, sailing some 100 km out to sea in international waters.

According to CNN, the submarine's activity was different than the typical training usually observed closer to shore.

The US military pays close attention to North Korean submarine activity following the 2010 Cheonan incident where a North Korean sub torpedoed a South Korean Naval vessel.

North Korea last year conducted its first successful submarine missile test, firing a missile called the the KN-11 or Pukguksong-1.