North Korea has fired two unidentified projectiles into its eastern sea as it begins to resume weapons demonstrations after a months-long hiatus that could have been forced by the coronavirus crisis in Asia.

Key points: Kim Jong-un has warned that the North is no longer bound by a suspension of missile tests

Kim Jong-un has warned that the North is no longer bound by a suspension of missile tests Experts speculate that Pyongyang wants to boost military morale amid COVID-19

Experts speculate that Pyongyang wants to boost military morale amid COVID-19 North Korea is yet to report any cases of coronavirus but some are thought to have been quarantined

The launches came two days after North Korea's state media said leader Kim Jong-un supervised an artillery drill aimed at testing the combat readiness of units in front-line and eastern areas.

Seoul's Joint Chiefs of Staff said the projectiles, fired from an area near the coastal town of Wonsan, flew about 240 kilometres north-east with an apogee (maximum altitude) of about 35 kilometres.

It said the South Korean and US militaries were jointly analysing the launches but didn't immediately confirm whether the weapons were ballistic or rocket artillery.

North Korea likely tested one of its new road-mobile, solid-fuel missile systems or a developmental "super-large" multiple rocket launcher it repeatedly demonstrated last year, said Kim Dong-yub, an analyst from Seoul's Institute for Far Eastern Studies.

Experts say such weapons can potentially overwhelm missile defence systems and expand the North's ability to strike targets in South Korea and Japan, including US bases there.

Kim Jong-un had entered the New Year vowing to bolster his nuclear deterrent in the face of "gangster-like" US sanctions and pressure, using a key ruling party meeting in late December to warn of "shocking" action over stalled nuclear negotiations with the Trump administration.

He also said the North would soon reveal a new "strategic weapon" and insisted the North was no longer "unilaterally bound" to a self-imposed suspension on the testing of nuclear and intercontinental ballistic missiles.

But Mr Kim did not explicitly lift the moratorium or give any clear indication that such tests were impending and seemed to leave the door open for eventual negotiations.

Mr Kim and President Donald Trump met three times since embarking on their high-stakes nuclear diplomacy in 2018, but negotiations have faltered since their second summit last February in Vietnam, where the Americans rejected North Korean demands for major sanctions relief in exchange for a partial surrender of its nuclear capability.

South Korea's military says North Korea has fired unidentified projectiles. ( AP: Korean Central News Agency/Korea News Service )

Speculation over need to boost morale amid COVID-19

The recent lull in North Korea's launches had experts wondering whether the North was holding back its weapons displays while pushing a tough fight against the coronavirus, which state media has described as a matter of "national existence".

Some analysts speculated that the North cut back training and other activities involving large gatherings of soldiers to reduce the possibility of the virus spreading within its military.

Mr Kim's latest show of force is apparently aimed at boosting military morale, strengthening internal unity and showing that his country is doing fine despite outside worries of how the North would contend with an outbreak.

North Korea in previous years has intensified testing activity in response to springtime military exercises between South Korea and the United States while describing them as invasion rehearsals.

But the allies announced last week that they were postponing their annual drills due to concern about the coronavirus outbreak in South Korea that infected soldiers of both countries.

The launches were the latest setback for dovish South Korean President Moon Jae-in, who despite the North's indifference has repeatedly pleaded for reviving inter-Korean engagement.

In a speech on Sunday marking the 101st anniversary of a major uprising against Japanese colonial rule, Mr Moon called for cooperation between the Koreas to fight infectious disease amid the COVID-19 outbreak in Asia.

Amid the deadlock in larger nuclear negotiations with the Trump administration, Mr Kim has suspended virtually all cooperation with South Korea in past months while demanding that Seoul defies US-led international sanctions and restart inter-Korean economic projects that would jolt the North's struggling economy.

North Korea has yet to confirm any COVID-19 cases, although state media has hinted that an uncertain number of people have been quarantined after exhibiting symptoms.

It has shut down nearly all cross-border traffic, banned tourists, intensified screening at entry points and mobilised tens of thousands of health workers to monitor residents and isolate those with symptoms.

South Korea last month withdrew dozens of officials from an inter-Korean liaison office in Kaesong after North Korea insisted on closing it until the epidemic is controlled.

AP