North Korea on Tuesday continued to ramp up its weapons demonstrations by firing unidentified projectiles twice into the sea while lashing out at the United States and South Korea for continuing their joint military exercises that the North says could derail fragile nuclear diplomacy.

South Korea's military alerted reporters of the launches minutes before an unidentified spokesperson of the North's Foreign Ministry released a statement denouncing Washington and Seoul over the start of their joint exercises on Monday. The statement said the drills, which North Korea sees as an invasion rehearsal, leave the country "compelled to develop, test and deploy the powerful physical means essential for national defence."

The North's spokesperson said Pyongyang remains committed to dialogue, but it could seek a "new road" if the allies don't change their positions.

"It is too axiomatic that a constructive dialogue cannot be expected at a time when a simulated war practice targeted at the dialogue partner is being conducted," the spokesperson said in a statement released by Pyongyang's official Korean Central News Agency. "We remain unchanged in our stand to resolve the issues through dialogue. But the dynamics of dialogue will be more invisible as long as the hostile military moves continue."

Seoul's Joint Chiefs of Staff said the projectiles were launched early Tuesday from an area near the North's western coast and flew cross-country before landing in waters off the country's eastern coast.

It didn't immediately say how many projectiles were fired or how far they flew.

South Korea's government had no immediate statement on the North's launches, which were its fourth round of weapons tests in less than two weeks. The office of South Korean President Moon Jae-in said his chief national security adviser, Chung Eui-yong, will hold an emergency meeting with the country's defence minister and spy chief on Tuesday to discuss the launches.

North Korea had said it will wait to see if the August exercises actually take place to decide on the fate of its diplomacy with the United States and whether to continue its unilateral suspension of nuclear and long-range missile tests, which leader Kim Jong-un announced last year amid a diplomatic outreach to Washington.

South Korean President Moon Jae-in is seen at the presidential Blue House in Seoul on Monday. (Park Jin-hee/Newsis/Associated Press)

South Korea's Defence Ministry on Monday didn't initially confirm the start of the joint military exercises before Defence Minister Jeong Kyeong-doo told a parliamentary hearing that the drills had begun. Jeong did not provide specific details about the drills, which were expected to be computer simulated and not involve actual combat troops and equipment.

The North last week conducted two test firings of what it described as a new rocket artillery system and carried out a short-range ballistic missile launch on July 25, which it described as a "solemn warning" to South Korea over its plans to continue military drills with the United States. Experts say the North's weapons display could intensify in the coming months if progress isn't made on the nuclear talks.

The North's launches came a day after Moon made a nationalistic call for economic co-operation between the Koreas while denouncing Japan's imposition of trade curbs on the South amid an escalating diplomatic row.

'Peace economy'

Moon's insistence that a "peace economy" by the Koreas would be able to erase Japan's comparative economic superiority by "one burst" drew instant criticism from conservatives, who accused the president of ignoring the North's tests of short-range weapons that experts say pose a serious threat to the South's security.

Even if economic co-operation between the Koreas fully resumes after quick progress in nuclear diplomacy — which looks increasingly unlikely — rebuilding the North's dismal economy following decades of isolation and policy blunders could be a long and excruciating process.

Moon met with Kim three times last year, and they agreed to resume economic co-operation when possible, voicing optimism that international sanctions could end to allow such activity. But the bilateral peace process between the Koreas has ground to a halt amid the stalemate in larger nuclear negotiations with the United States.

The North has demanded that Seoul turn away from Washington and restart inter-Korean economic projects held back by U.S.-led sanctions against the North.

The Trump administration says the sanctions will be fully enforced until the North makes concrete steps toward relinquishing its nuclear weapons and ballistic missile program.

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The allies have scaled down their major military exercises and stopped regional dispatches of U.S. strategic assets such as long-range bombers and aircraft carriers since the first summit between Kim and President Donald Trump in June 2018 in Singapore to create space for diplomacy.

The North insists even the downsized drills violate agreements between Kim and Trump, who in Singapore vowed to improve bilateral ties and issued a vague statement on a nuclear-free Korean Peninsula without describing when and how it would occur.

Nuclear negotiations have been at a standstill since the collapse of the second Trump-Kim summit in Vietnam in February over disagreements in exchanging sanctions relief and disarmament.

The North's recent weapons tests have dampened the optimism that followed the third summit between Trump and Kim on June 30 at the inter-Korean border. The leaders agreed to resume working-level nuclear talks that stalled since February, but there have been no known meetings between the two sides since then.