Fears Kim Jong-un is ready to launch a nuclear weapon are increasing as statements from the country have become more direct and aggressive following an announcement on Friday by US Secretary of State Rex Tillerson the period of diplomatic patience with the dictatorship is officially over.
In South Korea, he outlined a tougher strategy to confront North Korea's nuclear threat.
The North said on Sunday that it recently tested a new high-thrust rocket engine, raising speculation that Pyongyang might be seeking near-future provocations despite warnings by the global community.
Earlier this month, North Korea fired off four ballistic missiles into the Sea of Japan, reportedly reaching within 120 miles of Japan's shoreline.
Mr Tillerson said the USA did not want a military conflict, but if North Korea took actions that threatened South Korean forces or U.S. forces, "that would be met with [an] appropriate response".
When asked about the possibility of using military force to combat the threat from North Korea, Mr Tillerson told a news conference in South Korea's capital Seoul: "All of the options are on the table". Meanwhile, the New York Times reported that any pre-emptive strike against North Korea, as mused by the Trump administration, would entail serious challenges and risks.
During Tillerson's visit to China, the two sides said they would work together to try to make North Korea take "a different course".
The North's KCNA news agency said on Sunday the engine would help North Korea achieve world-class satellite-launch capability, indicating a new type of rocket engine for an intercontinental ballistic missile (ICBM).
North Korea has accelerated its weapons development, violating multiple UN Security Council resolutions and appearing undeterred by tough global sanctions.
The United States has long called on China to do more to rein in its ally, North Korea.
China has also been infuriated by the deployment of an advanced USA anti-missile system in South Korea, which it says will both harm China's own security and do nothing to ease tensions.