North Korea vowed to wage a "merciless, sacred war" against the United States on Thursday, days before the launch of annual joint South Korea-US military exercises that have incensed Pyongyang.

"Nuclear weapons are not a monopoly of the US," the ruling party's official newspaper, Rodong Sinmun, warned in an editorial carried by the state KCNA news agency.

"The US is seriously mistaken if it thinks its mainland is safe," the editorial added.

North Korea "will wage a merciless sacred war against the US now that the latter has chosen confrontation", the Rodong Sinmun said.

North Korea regularly ramps up the bellicose rhetoric before the start of the annual joint military exercises that always see a sharp surge in tensions on the divided peninsula.

Seoul and Washington insist they are defensive in nature, but they are condemned by Pyongyang as provocative rehearsals for invasion.

North Korea had offered a moratorium on nuclear testing if this year's joint drills were cancelled -- a proposal rejected by Washington as an "implicit threat" to carry out a fourth nuclear test.

The editorial came as South Korea and the United States conducted a joint naval drill Friday, involving 10 South Korean warships and a US Aegis destroyer.

The drill was a prelude to the eight-week Foal Eagle exercise which kicks off Monday and involves air, ground and naval field training, with around 200,000 Korean and 3,700 US troops.

A week-long, largely computer-simulated joint drill, Key Resolve, also gets underway Monday.

North Korea has made threats against the US mainland before, although it has never demonstrated a missile strike capability of that range.

Although its nuclear program remains shrouded in uncertainty, Pyongyang is currently believed to have a stockpile of some 10 to 16 nuclear weapons fashioned from either plutonium or weapons-grade uranium.

A new research report by US experts published this week estimated that North Korea could be on track to have an arsenal of 100 nuclear weapons by 2020.