China's President Xi Jinping has told US President Donald Trump there needs to be a peaceful resolution to the North Korean nuclear issue.

Key points: Xi urged peaceful solution just hours after Trump boasted US was "locked and loaded"

Xi urged peaceful solution just hours after Trump boasted US was "locked and loaded" Relationship between two presidents is "extremely close," says White House

Relationship between two presidents is "extremely close," says White House N.Korea says nearly 3.5 million people have volunteered to join its army

In a telephone call with Mr Trump, the Chinese President urged all sides to avoid words or action that raised tensions.

Mr Xi's comments came hours after Mr Trump warned North Korea that the US military was "locked and loaded" and Pyongyang accused the US leader of driving the Korean peninsula to the brink of nuclear war.

The Pentagon said the US and South Korea would proceed as planned with a joint military exercise in 10 days, an action sure to further antagonise North Korea.

China's foreign ministry said in a statement that Mr Xi told Mr Trump a peaceful resolution to the North Korean nuclear issue was essential, and urged calm.

"Concerned parties must exercise restraint and avoid remarks and actions that escalate tensions on the Korean peninsula," it cited Mr Xi as saying.

In their phone call, Mr Trump and Mr Xi "agreed North Korea must stop its provocative and escalatory behaviour," the White House said in a statement, and reiterated their mutual commitment to denuclearise the Korean peninsula.

It added the relationship between Mr Trump and Mr Xi was "extremely close" and "will hopefully lead to a peaceful resolution of the North Korea problem."

Mr Trump, vacationing at his Bedminster, New Jersey, golf resort, earlier took to Twitter to warn North Korean leader Kim Jong-un that US "military solutions are now fully in place, locked and loaded, should North Korea act unwisely".

Again referring to Kim, Trump added, "If he utters one threat ... or if he does anything with respect to Guam or anyplace else that's an American territory or an American ally, he will truly regret it, and he will regret it fast."

In remarks to reporters on Friday after a meeting with US Secretary of State Rex Tillerson and US Ambassador to the United Nations Nikki Haley, Mr Trump said the situation with North Korea was "very dangerous and it will not continue".

He added, "We will see what happens. We think that lots of good things could happen, and we could also have a bad solution."

Despite the tough rhetoric, Mr Trump insisted that "nobody loves a peaceful solution better than President Trump."

South Korea's presidential Blue House said in a statement on Saturday the US and China were working to resolve the North Korea crisis, and it hoped the two leaders' phone call "will be able to resolve the peak of tension and act as a catalyst for the situation to move on to a new dimension."

Millions volunteer for People's Army, says North Korea

North Korea said nearly 3.5 million workers, party members and soldiers volunteered to join or re-join its army to resist new UN sanctions and to fight against the United States in the current geopolitical tension between Pyongyang and Washington.

Rodong Sinmun, North Korea's official newspaper, said the volunteers had offered to join or re-join the People's Army after the Korea Central News Agency (KCNA) issued a statement condemning new sanctions imposed by the United Nations in retaliation for North Korean missile tests.

Earlier this week, nuclear-armed North Korea threatened to strike the United States and its Pacific territory of Guam.

KCNA said a mass rally was held in Pyongyang to support the government.

North Korea has previously mobilised large crowds to show its resolve when tensions escalate.

In August 2015, one million North Koreans offered to enlist or re-enlist in the army when a mine exploded in the demilitarised zone between the two Koreas, raising additional tensions.

North Korea warned foreign diplomats to leave Pyongyang in 2013 when it suspended work at a joint inter-Korean industrial park and threatened missile strikes on US Pacific bases, notably in Guam and Hawaii.

The US Pacific territory of Guam is located in the Pacific Ocean. ( ABC News )

Reuters