The two governments hope to achieve North Korea's abandonment of its nuclear program through heightening US military pressure as well as strengthening economic sanctions by China.
Any preemptive action by the U.S. will only amplify South Korea's worry.
China made a plea for a return to negotiations.
However President Trump has also repeatedly warned he would "solve" North Korea if China decided not to help.
That kind of rhetoric has unnerved allies in Japan and South Korea, who would be at the sharp end of any North Korean response. On Wednesday, Secretary of State Rex Tillerson said the US wants to force North Korea leader Kim Jong Un to the negotiating table. As a result, US planners are not taking any chances when it comes to stepping-up missile defense technology and preparedness.
A preemptive strike by the USA on North Korean missile facilities is not a practical solution, unless there is a strong belief that the North Koreans will attack the US.
That's not all, North Korea also regularly threatens Japan.
Some see him as a hot-tempered, unpredictable leader who might attack North Korea before it masters the technology to build a nuclear-tipped missile that could hit the US mainland. It could be a deliberate ploy, a subtle warning to North Korea, but beyond that there seems to be no strong reason for a strike.
At the beginning of the meeting, Abe emphatically said: "Unlike the former administration of U.S. President Barack Obama, which pursued a policy of 'strategic patience, ' the administration of Donald Trump is trying to deal with the problem by thinking that all options are on the table". Both Japan and South Korea are home to tens of thousands of USA troops. Further missile tests followed in March, along with an assertion by US Secretary of State Rex Tillerson that the US would not rule out a pre-emptive strike on North Korea to counter the nuclear threat. According to analysts, during that parade a submarine launched ballistic missile was shown for the first time.
Ruling party defense experts have even proposed that Japan lift a self-imposed restraint on conducting a retaliatory strike if attacked, rather than relying exclusively on the US military. Otherwise it risks its alliance with South Korea and Japan.
That's right, Kim Jong-un is taking no prisoners and has warned Americans "Don't mess with us" on North Korean state media. Tokyo is increasingly concerned about this possibility.