As the new emerging powers - India, Brazil, Japan and Germany - made a strong case for UNSC reforms mobilising international opinion, Chinese Vice Foreign Minister He Yafei told a US diplomat that Beijing "was concerned by the 'momentum' that was building on UN Security Council reform," the leaked cable said.
He told the US diplomat that the position enjoyed by current P-5 - China, Britain, France, Russia and United States - should not be "diluted" by making it P-10.
"If we end up with a 'P-10,' both China and the United States would be in trouble," US charge d'affaires in Beijing, Dan Piccuta quoted He as saying in his cable.
"Moreover, it would be difficult for the Chinese public to accept Japan as a permanent member" of the Security Council due to historic discard between the two countries and asked Washington not to be "proactive" about it.
The US diplomat, speaking in April 2009, replied that "no decision had been made about which countries to support for permanent membership."
"It was hard to envision any expansion of the Council that did not include Japan, which was the second-largest contributor to the UN budget," he said.
For its part US endorsed only Japan when Tokyo along with India, Brazil and Germany made the bid, which ended up creating a major debate in the international forum, which some other countries including Pakistan launching strong campaigning against it.
China, which keeps repeating a standard line that it understands India's aspirations to play a bigger, apparently was caught unawares when US President Barrack Obama endorsed New Delhi's candidature during his last month's visit.
Chinese officials told their Indian counterparts during recent talks that Obama's endorsement came as a surprise to them following which Beijing has said it was willing to discuss with India on the UNSC reform process.
Chinese Premier Wen Jiabao was expected to provide greater clarity to India's case for UNSC permanent seat during his visit to India in the middle of this month.