The comments by an unnamed North Korean foreign ministry spokesman, carried by state news agency KCNA, came as Pyongyang fired two "unidentified projectiles" off its east coast, South Korea's Yonhap news agency reported, citing the Joint Chiefs of Staff.

"Despite our repeated warnings, the United States and South Korean authorities have finally started the joint military exercise targeting the DPRK," the foreign ministry spokesman for the nuclear-armed North said, according to KCNA.

"This is an undisguised denial and a flagrant violation of June 12 DPRK-US Joint Statement, Panmunjom Declaration and September Pyongyang Joint Declaration, all of which are agreements to establish new DPRK-US relations and build a lasting and stable peace regime on the Korean peninsula," the spokesman said.

After a year of mutual threats and mounting tension, US President Donald Trump and the North's Kim Jong Un held a historic summit in Singapore last year, when Kim signed a vague pledge to work towards "denuclearisation of the Korean peninsula".

A second summit in Hanoi in February broke up amid disagreement on sanctions relief and what the North might be willing to give up in return.

Trump and Kim agreed to resume nuclear talks during an impromptu June meeting in the Demilitarized Zone that divides the peninsula, but that working-level dialogue has yet to begin.

Pyongyang warned last week that further nuclear talks could be derailed if Seoul and Washington push ahead with the manoeuvres, which began on Monday.

The North's reported firing on Tuesday comes after three earlier weapons tests over eight days.