China's UN ambassador Wang Guangya said he reached the agreement with John Bolton during a meeting on Tuesday -which was the new US ambassador's first full day in his new post - because both believe the proposal by the so-called Group of Four would divide the UN's 191 member states.
Washington and Beijing are already on record as opposing the G-4 resolution for different reasons, but the agreement would mark a new joint effort to prevent its approval by the UN General Assembly, which requires a two-thirds "yes" vote.
Wang has said the ultimate objective of China and the United States is to expand the Security Council with a formula that is not divisive.
"But at this stage, I think our objective will be to oppose the G-4, to make sure they do not have sufficient votes to take the risk to divide the house," he said on Wednesday.
After 10 years of seemingly endless debate, Secretary-General Kofi Annan told UN member states in March that he wanted a decision on Security Council expansion before a summit of world leaders in September.
The US-China effort to defeat the Group of Four comes on the eve of Thursday's emergency summit, called by the African Union, to consider whether to approve a compromise agreement, which some of its ministers reached with Brazil, Germany, India and Japan in London on 25 July.
Bolton (L) agreed with the Chinese
envoy to oppose the G-4 proposal
Brazil, Germany, India and Japan have introduced a resolution calling for a 25-member council that would add six permanent seats without a veto and four nonpermanent seats.
They are hoping to win four of the permanent seats with the other two earmarked for Africa.
South Africa, Nigeria and Egypt are the leading African contenders.