BEIJING (Reuters) - China's Foreign Ministry said on Monday (Sept 12) that sanctions alone cannot solve the North Korean nuclear issue and that unilateral action can only lead to a dead end.

The crux of the issue lies with the United States, not China, and China has made many efforts to uphold peace and stability on the Korean Peninsula, ministry spokesman Hua Chunying told a daily news briefing in Beijing.

Meanwhile, Japanese Prime Minister Shinzo Abe told senior military officials on Monday that he absolutely cannot tolerate North Korea having conducted two nuclear tests since the start of the year.

North Korea on Friday conducted its fifth and biggest nuclear test and said it had mastered the ability to mount a warhead on a ballistic missile, ratcheting up a threat that rivals and the United Nations have been powerless to contain.

"North Korea went ahead with two nuclear tests in just nine months. This is absolutely unacceptable," Mr Abe said.

South Korea's Defence Ministry on Monday said North Korea is ready to conduct an additional nuclear test at any time.

"Assessment by South Korean and US intelligence is that the North is always ready for an additional nuclear test in the Punggye-ri area," the site of the North's five nuclear explosions, South Korean Defence Ministry spokesman Moon Sang Gyun told a news briefing.

"North Korea has a tunnel where it can conduct an additional nuclear test," he said.

North Korean Foreign Minister Ri Yong Ho, formerly the country's chief nuclear negotiator, arrived in Beijing on Monday and was seen entering the country's embassy, Japan's Kyodo news agency reported.

Mr Ri left Pyongyang to attend a meeting of the Non-Aligned Movement countries in Venezuela and later the UN General Assembly, the Associated Press reported from Pyongyang without citing a source.

His trip comes amid a fresh push by the United States and South Korea for more sanctions following the nuclear test.

A US special envoy for the isolated state, Mr Sung Kim, will travel to Seoul on Monday. Mr Kim met Japanese officials on Sunday and said the United States may launch unilateral sanctions against North Korea, echoing comments by US President Barack Obama on Friday in the wake of the test.

A push for further sanctions was "laughable", North Korea said on Sunday, vowing to continue to strengthen its nuclear power.