Diplomatic efforts to resolve the crisis over North Korea's missile tests appear to have made little headway with two sets of talks failing to progress.
Mr Hill said talks in Pyongyang appeared to have failed
Soon after a US envoy said China-North Korea talks had not had a breakthrough, inter-Korean talks collapsed.
But key nations remain at odds on what action to take, setting the scene for a showdown in the UN Security Council.
China says it will veto a Japanese resolution on the issue and, with Russia, has circulated a rival draft.
Japan is seeking a tough resolution - backed by the US, UK and France - that could lead to sanctions against North Korea. But China and Russia have proposed a softer draft that emphasises a diplomatic solution.
The Security Council decided to delay action to allow time for key talks between China and North Korea.
Officials were hoping that China, as North Korea's closest ally, could persuade Pyongyang not to repeat last week's missile launches and return to six-party talks on its nuclear ambitions.
But US envoy Christopher Hill said on Thursday that it was clear these talks had failed.
He told the BBC from Beijing, where he has been holding discussions over the North's missile launches, that there had been no breakthroughs in Pyongyang.
Mr Hill told reporters the Chinese were "as baffled as we are".
"The Chinese sent a good delegation up to Pyongyang, showed a real interest in trying to work with the DPRK [North Korea] - but it does not appear to have been reciprocated."
South Korea, meanwhile, has also been attempting to negotiate with the North, at high-level bilateral talks in the port city of Busan.
But the talks ended in failure one day ahead of schedule, with the North Korean delegation set to return to Pyongyang.
The North blamed South Korea for the collapse, saying that Seoul had raised issues unrelated to the meeting.
"The South side will pay a price before the nation for causing the collapse of the ministerial talks," a statement from the North Korean delegation said.
Meanwhile Chinese delegates are due to remain in Pyongyang until Saturday, continuing their quest for progress.
But Mr Hill, who is soon to fly back to Washington, said the time left for Chinese efforts to work was "down to a number of days".
On 5 July North Korea test-fired seven missiles - including a long-range Taepodong-2, a weapon which is believed to be capable of reaching Alaska.
The action has left the international community deeply divided over how to respond.
The UN resolution drawn up by Japan - and backed by the US, Britain, France, and five other members - calls North Korea a "threat to international peace and security" and invokes Chapter Seven of the UN charter.
Mr Wang said he had been told to veto the draft resolution
Resolutions made under Chapter Seven are legally binding and can authorise sanctions or even military action.
But China, Russia and South Korea believe punitive action is not necessary, and China's ambassador to the UN, Wang Guangya, confirmed on Thursday he had been instructed to veto the resolution at the Security Council.
A rival Chinese-Russian draft instead focuses on the need to find a diplomatic solution.
The US and Japan said the resolution had serious gaps, and that they would press for a vote on their resolution.
Japan's chief spokesman, Chief Cabinet Secretary Shinzo Abe, told reporters on Thursday that Tokyo would not wait forever for a Security Council vote.
"I will not give a deadline now, but there should be a vote early as possible," Mr Abe said.