North Korea said on Tuesday it had ordered military units to fire more artillery shells into the sea, the North's official KCNA news agency reported, in response to South Korea's ongoing drills across the border.

The firing comes a day after North Korea said it fired more than 130 shells into the sea off its east and west coasts, some of which landed in a buffer zone near the sea border between the two Koreas.

Seoul called that a violation of a 2018 inter-Korean agreement to reduce tensions.

South Korean and US troops have been conducting live-fire drills near the border since Monday.

The allies said the drills are necessary to deter nuclear-armed North Korea, which tested a record number of missiles this year and has made preparations to resume nuclear testing for the first time since 2017.

Pyongyang has sharply criticised the joint exercises as provocative and proof of hostile policies by Seoul and Washington.

"The enemy must immediately cease provocative military actions in the zone near the front lines," a spokesperson for the North Korean army said, according to KCNA.

In addition to the artillery fire, the North Korean army issued a combat emergency alert to units at all levels and troops were ordered to step up surveillance, the spokesperson said.

- Reuters