Secretary of State Rex Tillerson has said during a visit to South Korea that a military action against North Korea is “an option on the table”.
He said that the policy of “strategic patience” had ended and the US was exploring a new range of diplomatic, security and economic measures.
Rex Tillerson also defended the deployment of a US missile defense system in South Korea – a move that has angered Beijing.
Seoul and Washington say the system is needed to defend against North Korea.
The secretary of state arrived in South Korea from Japan, where he said that 20 years of efforts aimed at persuading North Korea to abandon its nuclear ambitions had failed.
Asked if the possibility for military action existed, Rex Tillerson said: “Certainly we do not want to, for things to get to military conflict.”
However, he added: “If they elevate the threat of their weapons program to a level that we believe requires action, then, that option’s on the table.”
Rex Tillerson also called on China to fully implement sanctions imposed by the UN in response to North Korea’s nuclear and missile tests.
He said: “I don’t believe we have ever fully achieved the maximum level of action that can be taken under the UN Security Council resolution with full participation of all countries.”
The US accuses China, North Korea’s main ally, of not doing enough to rein it in. However, Beijing remains wary of any action that could destabilize the North Korean regime and potentially create chaos on its border.
China is also strongly opposed to the deployment of the Terminal High Altitude Area Defense (THAAD) system in South Korea.
The US says the system is aimed at countering the threat North Korea, but China says its powerful radar will allow the US to spy on its territory.
In recent days there have been multiple reports of apparent economic retaliation aimed at South Korea by Beijing.
Secretary Rex Tillerson called these actions “unnecessary and troubling”.