US Vice-President Mike Pence has said his country's "era of strategic patience" with North Korea is over.
Pence made the remarks at the demilitarised zone (DMZ), the area dividing the two Koreas, during a visit to South Korea to reaffirm ties.
His visit comes amid escalated tensions on the peninsula, with heated rhetoric from both North Korea and the US.
He arrived in Seoul on Sunday hours after North Korea carried out a failed missile launch.
On Monday, the US and South Korea launched a joint air force military exercise to ensure readiness against North Korea, according to South Korean media.
Pence, whose father served in the Korean War, was speaking on Monday at the truce village of Panmunjom, where the war's armistice was signed.
He told reporters: "There was a period of strategic patience, but the era of strategic patience is over."
The US wants to achieve security on the peninsula "through peaceable means, through negotiations", he said, "but all options are on the table".
Pence's latest comments echoed those made by Secretary of State Rex Tillerson, who warned that pre-emptive military action was "on the table" when he visited the DMZ last month.
Pence also reiterated the US commitment to South Korea, saying it was an "iron-clad alliance", and that North Korea "should not mistake the resolve" of the US to stand with its allies.
He has denounced North Korea's latest ballistic missile test as a "provocation".
Also on Monday Pence went to Camp Bonifas, a United Nations military compound near the DMZ.
Pence is visiting South Korea, Japan, Indonesia and Australia on a 10-day Asia tour.
On Sunday, Lt Gen HR McMaster, the US top security adviser, said his country was working on a "range of options" with China, the first confirmation the two countries were co-operating to find a solution to the North Korean issue.
US President Donald Trump also said on Sunday that Beijing was "working with us on the North Korean problem". He had stated last week that the US and its allies may "deal with" Pyongyang if China did not.
The BBC's Stephen Evans in Seoul says US policy now seems to persuade China to contain North Korea while keeping the economic and military pressure on.
China, historically Pyongyang's sole major ally, has reiterated its call for North Korea to stop all tests, and has also called for a peaceful solution.
Besides Sunday's launch, North Korea has held a series of large-scale events in the past week including a massive celebration and military parade on Saturday.
It has denounced the US deployment of an aircraft carrier group to the region, saying it would respond by "force of arms" to "reckless moves".
Observers have said North Korea may conduct a sixth nuclear test soon, with activity reported at nuclear facilities, according to the website 38 North.
South Korea has sent about 500 personnel and planes. The Max Thunder exercise will last for two weeks.
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