South Korea on Wednesday suspended a clause in the agreement and said it would immediately step up military surveillance along the heavily fortified border with North Korea.

In a statement carried by the KCNA news agency, North Korea's defence ministry said it would restore all military measures it had halted under the deal aimed at de-escalating tension between the two Koreas.

"We will forward deploy stronger armed forces and new military equipments near the military demarcation line," the statement said. "South Korea will be held entirely responsible if an irrevocable clash occurs between the North and the South."

The suspended North-South pact, known as the Comprehensive Military Agreement, was signed at a 2018 summit between North Korean leader Kim Jong Un and then South Korean President Moon Jae-in.

Critics have said the pact weakened Seoul's ability to monitor North Korea while Pyongyang had violated the agreement.

North Korea's statement came hours after it fired a ballistic missile toward the sea east of the Korean peninsula late on Wednesday. South Korea's military said the launch appeared to have failed.

(Reporting by Soo-hyang Choi; editing by Diane Craft and Deepa Babington)