President Donald Trump has warned the US patience with North Korea's "reckless and brutal regime" has run out and called on the international community to back additional sanctions to thwart the North's nuclear and ballistic missile programmes.
During his remarks at the White House alongside South Korean President Moon Jae-in on Friday, Trump also vowed to "strengthen" the US-South Korea alliance.
"The era of strategic patience with the North Korean regime has failed," said Trump. "Frankly, that patience is over.
North Korea issue dominates talks at Asia Security Summit
"Our goal is peace, stability and prosperity for the region. But the United States will defend itself - always will defend itself. Always. And we will always defend our allies," he said.
Moon, who has been more dovish than his predecessors on the issue of the North, nonetheless echoed Trump, promising a "stern response" to any further provocations.
"Our two leaders will employ both sanctions and dialogue in a phased and comprehensive approach," Moon said.
Pyongyang views American engagement on the Korean Peninsula - including the presence of 30,000 troops - as a signal Washington is preparing to reignite the 1950-53 Korean War.
US Defense Secretary Jim Mattis warned the consequences of any military solution would be "tragic on an unbelievable scale".
Trump had pinned his hopes on persuading China, North Korea's neighbour and main trading partner, to do more to rein in Pyongyang, although he has lately grown frustrated that Beijing has not taken stronger action.
On Thursday, the US targeted a Chinese bank and sanctioned Chinese individuals and a firm for dealing with North Korea and approved a $1.42bn arms deal with Taiwan - decisions that angered Beijing.
During his remarks, Trump also said he would work toward leveling the US trade imbalance with Seoul and seek a trade "deal that's fair for the United States and fair for South Korea".
Trump said the United States was renegotiating what he characterised as a "rough" trade deal with South Korea agreed to five years ago by his predecessor, Barack Obama.
"We will do more to remove barriers to reciprocal trade and market access," Trump said, adding that the two leaders had talked about the thorny trade areas of steel and autos. 