North Korea has threatened "indiscriminate" nuclear strikes on the US and South Korea as the two begin their largest ever military drills.
The exercises, Key Resolve and Foal Eagle, are an annual event and always generate tension.
The order for a "pre-emptive nuclear strike of justice" was made in a statement put out by Pyongyang.
Such rhetoric is not uncommon, and experts doubt the North's ability to put nuclear warheads on its missiles.
North Korea says it sees the annual US-South Korean war games as a rehearsal for invasion. Last year, it threatened to turn Washington into a "sea of fire".
"We will launch an all-out offensive to decisively counter the US and its followers' hysteric[al] nuclear war moves," a newsreader on the state-run North Korean KRT news channel said of the latest exercises.
Approximately 17,000 US forces are participating in the exercises, alongside around 300,000 South Korean troops - both significant increases on 2015's numbers.
Despite starting on the same day, Key Resolve is more computer simulation-driven and ends on 18 March, while Foal Eagle is more focussed on field exercises and runs until 30 April.
The South's defence ministry has warned Pyongyang against any "rash act that brings destruction upon itself".
"If North Korea ignores our warning and makes provocations, our military will firmly and mercilessly respond to it," said spokesman Moon Sang-gyun.
Japan's foreign minister Fumio Kishida also demanded that North Korea show restraint.
"North Korea's nuclear, and nuclear missile, development is absolutely unacceptable. We will coordinate with the international community to demand that North Korea show restraint, and abide by the various resolutions including that of the six party talks," Mr Kishida said.
Though unconfirmed, South Korean news agency Yonhap, citing military sources, has reported that the exercises will include training for precision attacks on North Korean leadership and its nuclear and missile facilities.
These latest exercises come just days after the UN passed new sanctions against North Korea following its recent nuclear test and rocket launch.
On Saturday, the Philippines impounded a North Korean cargo vessel under the toughened measures. A presidential spokesman said the crew would be deported and the ship subject to a UN-mandated inspection.
The North responded to the sanctions by saying it was readying nuclear weapons for "pre-emptive" use, and by firing short-range missiles into the sea.
Seoul is expected to announce more sanctions of its own on Tuesday, which is likely to draw another angry response from Pyongyang.
The US and South Korea on Friday also began formal talks on the deployment of a US missile defence system to the peninsula, a move strongly opposed by North Korea, Russia and China.
Beijing says the Thaad anti-missile system compromises its security and would undermine its nuclear deterrent.