US President Donald Trump says "only one thing will work" in dealing with nuclear-armed North Korea, saying previous administrations had talked to Pyongyang without results.
"Presidents and their administrations have been talking to North Korea for 25 years, agreements made and massive amounts of money paid," Mr Trump said in a tweet on Sunday morning.
"… Hasn't worked, agreements violated before the ink was dry, making fools of US negotiators. Sorry, but only one thing will work!"
Mr Trump did not make clear what he was referring to, but his comments seemed to be a further suggestion military action was on his mind.
The President has previously said the United States would "totally destroy" North Korea if necessary to protect itself and its allies from Pyongyang's nuclear threats.
In those comments, made during his first speech to the UN General Assembly, Mr Trump also mocked Pyongyang's leader Kim Jong-un as a "rocket man on a suicide mission".
Mr Trump's tweets follow comments earlier this week, when during a meeting with top US military leaders and their spouses, he told reporters it was the "calm before the storm".
Asked for clarification on what he meant, Mr Trump said: "You'll find out."
White House spokeswoman Sarah Sanders referred to Iran and North Korea the following day when asked about Mr Trump's comments.
Asked about Mr Trump's tweet, the Pentagon said the Defence Department's job was to "present the President military options and carry out orders".
Rhetoric raising the risk of slide into catastrophic war
Mr Trump repeatedly has made clear his distaste for dialogue with North Korea.
Last week he dismissed the idea of talks as a waste of time, a day after Secretary of State Rex Tillerson said Washington was maintaining open lines of communication with Kim Jong-un's Government.
The increasingly heated rhetoric between Mr Trump and North Korean leader Kim Jong-un is raising fears of a risk of a miscalculation by one side or the other that could have massive repercussions.
Last month North Korea's Foreign Minister Ri Yong Ho told the UN General Assembly that targeting the US mainland with its missiles was inevitable because of the comments from "Mr Evil President".
Mr Trump responded in kind on Twitter.
"Just heard Foreign Minister of North Korea speak at UN. If he echoes thoughts of Little Rocket Man, they won't be around much longer!" he wrote.
Australian Prime Minister Malcolm Turnbull, asked about Mr Trump's comments, said the global community needed to maintain economic pressure on North Korea.
"What we need to do is to continue to put that economic pressure as a global community on North Korea, and that is the best prospect of changing this regime from its dangerous and reckless course without conflict," he said.
Missiles — and threats — flying
The verbal warfare has come amid North Korea escalating its missile testing program, as its works to develop nuclear-armed missiles capable of hitting the US mainland.
North Korea has launched dozens of missiles this year, several flying over Japan.
On September 3, North Korea successfully tested a hydrogen bomb designed to be mounted on its newly developed intercontinental ballistic missile (ICBM), according to state television.
State television described the test, ordered by Mr Kim, as a "perfect success" and a "meaningful" step in completing the country's nuclear weapons program.
It was the sixth nuclear test by Pyongyang since 2006, and South Korea estimated it was five to six times stronger than North Korea's fifth test a year ago.
The North has also threatened to target Guam, a major regional base for the US military.
The United States has responded by increasing naval drills in the region and flying bombers over the Korean peninsula.
But the joint US-South Korea-Japan flyovers have prompted more threats from the North.
"Since the United States declared war on our country, we will have every right to make countermeasures, including the right to shoot down United States strategic bombers even when they are not inside the airspace border of our country," Ri Yong Ho told reporters in New York last month.