It is a make or break stage for negotiations over North Korea's nuclear programme, delegates at the six-nation talks in Beijing have said. Delegates are discussing a Chinese proposal to allow North Korea to use power produced by nuclear energy.
"We will have important meetings today that will determine either the success or failure of the talks," said Japan's chief delegate, Kenichiro Sasae.
The US envoy said he believed the draft proposals left too many ambiguities.
"We cannot create ambiguities at this stage that would cause confusion in the future," said US Assistant Secretary of State Christopher Hill.
The BBC's Charles Scanlon in Seoul says that after three years of confrontation, the US has indicated its patience is finally running out.
If no agreement can be reached under which North Korea gives up its suspected nuclear weapons programme, the US could refer it to the UN Security Council for potential sanctions, BBC News reports.