NEW YORK — US President Donald Trump warned North Korea on Thursday that there will be dire consequences for the reclusive country if it does not change its hostile attitude toward the United States.

"I will tell you this: North Korea better get their act together, or they’re going to be in trouble like few nations ever have been in trouble in this world," he told reporters at his golf club in New Jersey.

Trump made the comments after North Korea said earlier in the day it will develop a plan by mid-August to launch four missiles near the US Pacific territory of Guam, following the US leader’s threat to rain down "fire and fury" on the country.

"If North Korea does anything in terms of even thinking about attack of anybody that we love or we represent or our allies or us, they can be very, very nervous," he added.

Also Thursday, more than 60 members of the US Congress voiced "profound concern" over "irresponsible and dangerous" remarks made by Trump, in a letter addressed to Secretary of State Rex Tillerson.

They urged the top US diplomat to do all he can "to ensure that President Trump and other administration officials understand the importance of speaking and acting with the utmost caution and restraint on this delicate issue."

Defense Secretary Jim Mattis said "of course there’s a military option" but that "we want to use diplomacy" in resolving the issue of North Korea’s nuclear ambitions.

"What we’re doing is a diplomatically-led effort that is succeeding in drawing the international community together and speaking with one voice. You just saw it. That’s where we’re at," he told reporters.

Tensions between Washington and Pyongyang have escalated recently after North Korea tested two intercontinental ballistic missiles in July.

Subsequently, the UN Security Council imposed fresh sanctions on Pyongyang that aim to slash the country’s $3 billion annual export revenue by a third.

At the annual security meeting of Asia-Pacific foreign ministers in the Philippines on Monday, North Korea’s Foreign Minister Ri Yong Ho defended his country’s nuclear and missile programmes as legitimate and self-defensive in nature.

The Washington Post reported on Tuesday that North Korea has successfully produced a miniaturised nuclear warhead that can fit inside missiles, citing a confidential US assessment. — KYODO