South Korea's spy agency recently told lawmakers in a closed-door briefing that trade volume between North Korea and China in the first quarter declined 55 per cent from a year earlier. In March, the bilateral trade volume suffered a 91 per cent drop, legislators cited the agency as saying.

North took intense action to avoid an outbreak by closing its borders in January and quarantining thousands of people. It still contends it has had no cases of infection, though many outsiders doubt that.

A South Korean lawmaker who discussed the spy agency's findings earlier this week said the National Intelligence Service cannot rule out an outbreak in the North. But the agency concluded skyrocketing food prices and panic-buying in Pyongyang and a reduction in Kim's public appearances this year were evidence of the impact, the lawmaker said.

South Korea reported 12 fresh cases of the virus on Friday, its first increase above 10 in five days. Figures by the Korea Centres for Disease Control and Prevention brought national totals to 10,822 cases and 256 deaths.

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China announced one new virus case, a local infection in Jilin province, and 16 new cases of people who were not showing symptoms. Just 260 people remained hospitalised and 890 were in isolation as suspected cases or for having tested positive but without showing symptoms. China has reported 4633 deaths among 82,886 cases.

A North Korean military representative said military drills held on Wednesday were a grave provocation that demanded a reaction, according to a statement carried by KCNA.

"The recent drill served as an opportunity which awakened us once again to the obvious fact that the enemies remain enemies all the time," the statement said.

It cited a military exercise by the Republic of Korea Air Force (ROKAF) Air Combat Command on Wednesday, and said the drills violated inter-Korean agreements aimed at reducing military tensions.