North Korea lost a submarine, defense officials say

North Korea has a missing submarine, two defense officials told Fox News Friday.

One official told Fox the submarine is a 70-foot Yono class "midget" submarine that is diesel powered and has a crew of two, with room for a squad of saboteurs. It was unclear who may have been onboard.

North Korea does not operate large ballistic missile submarines, such as the U.S. Navy’s Ohio-class fleet.

Fox News has learned North Korea has not asked the United States to assist in search and rescue efforts.

On Thursday, North Korea responded to new sanctions from South Korea by firing short-range ballistic missiles into the sea in a show of defiance and vowing to "liquidate" all remaining South Korean assets at former cooperative projects in the North.

The moves are the latest in an escalating standoff between the Koreas that began in January when North Korea detonated what it said was an "H-bomb of justice," its fourth nuclear test.

Since then, the North has launched a long-range rocket and the South has shut a jointly-run factory park, slapped sanctions on the North, and begun large-scale war games with the United States.

North Korea responded by threatening nuclear strikes on South Korea and the U.S. mainland.

The missile firing came a day after North Korean media printed photos of what appeared to be a mock-up of a nuclear warhead.

Also, South Korea's spy agency told lawmakers Friday that the number of North Korean cyberattacks on the South has doubled over the past month.

On Monday at State of Air Force briefing at Pentagon, the Air Force's top officer, Gen. Mark Welsh, told Fox News that while the actions of North Korea are "very worrisome," the country does not have the capability to put a nuclear warhead on top of a missile and shoot it at the United States.

Hours later, North Korea warned of pre-emptive nuclear strikes after the United States and South Korea began holding their biggest ever war games.

This week Pentagon Press Secretary Peter Cook said he was confident that the U.S. military could knock out any North Korean ballistic missile in the sky.

Tensions remain high after North Korea's recent nuclear test and rocket launch, which prompted the United Nations to adopt tough new sanctions.

Fox News' Lucas Tomlinson and The Associated Press contributed to this report.