A senior US envoy is on an unscheduled visit to Beijing, amid Chinese opposition to the threat of sanctions.
And a Chinese team is in North Korea, expressing concern about the situation.
There are divisions at the UN on a Japanese draft resolution - backed by the US, France and UK - condemning North Korea.
North Korea raised tensions last week when it test-fired seven missiles - including a long-range Taepodong-2, a weapon which is believed to be capable of reaching Alaska.
In response, Japan drafted a resolution branding North Korea a "threat to international peace and security" and invoking Chapter Seven of the UN charter.
Resolutions made under Chapter Seven are legally binding and can authorise sanctions or even military action.
China, which proposed a statement condemning the tests but stopping short of sanctions, says the Japanese proposal is an "over-reaction" that would "increase tension".
The international community is angry at the North's missile tests
Seoul has also reacted with alarm over Japan's stance over North Korea, accusing Japan of acting recklessly, after Tokyo suggested the possibility of pre-emptive strikes on North Korean missile sites.
Amid these tensions, the flurry of diplomatic activity continues.
The American envoy to North Korea, Christopher Hill, is holding talks in Beijing, and a high-level North Korean delegation is in Seoul.
"Obviously, we're in a rather crucial period," Mr Hill told reporters at Beijing airport.
"The Chinese government has an important diplomatic mission going on and so we want to be in close consultation."
Meanwhile, Chinese President Hu Jintao told a visiting top North Korean official that China was opposed to any actions that "may worsen the situation on the Korean peninsula", Xinhua news agency reported.
The BBC's Richard Galpin, at the UN, says China and Russia, which both have the power of veto in the Security Council, believe that using a UN resolution to impose sanctions on North Korea at this stage would be irresponsible and unconstructive.
China's UN ambassador, Wang Guangya, said China was worried it could ultimately pave the way for military action against North Korea.
Instead, he said he believed the best initial response would be a non-binding statement by the Security Council calling on Pyongyang to stop the development of ballistic missiles and halt any testing.
That approach is backed by the Russian envoy to the UN, Vitaly Churkin, who said the statement provided "an excellent basis for a strong signal to Pyongyang" and "the right mode of action".
But the US, UK and France, also permanent members of the Security Council, say it is far too weak, especially as it is not legally binding.
All 15 members of the Security Council have at least agreed to hold off from voting immediately on a resolution calling for sanctions, to allow time for China to resolve the crisis through diplomatic means.
But after a meeting in Tokyo with the Japanese foreign minister, Christopher Hill expressed doubts about the extent of Beijing's influence over Pyongyang.
"I must say the issue of China's influence on the DPRK [North Korea] is one that concerns us, because China said to the DPRK: 'Don't fire those missiles' - and the DPRK fired them," he said.
The BBC's Charles Scanlon, in Seoul, says North Korea will take comfort from the widening gulf between its neighbours.
Last Mod: 20 Eylül 2018, 18:16