A top Chinese engineer who is building China's longest water tunnel in Yunnan province has said he "has not heard of" any plans to build a 1,000 km water diversion tunnel in Tibet for the Brahmaputra river.

His comments come a day after the Chinese government denied a media report which said engineers had submitted a plan in March to build the world's longest tunnel to divert the Brahmaputra's waters from Tibet to arid Xinjiang. The Chinese Foreign Ministry said it was "a false report".

The report, in the South China Morning Post, had quoted several Chinese experts who said that an on-going 600 km water tunnel in Yunnan was being used as "a demonstration project" for the Tibet Brahmaputra tunnel. But the chief engineer of the Yunnan plan denied this on Wednesday.

"There is no such direction from the central government, and I've never heard of any plan laid out for a Tibet-Xinjiang tunnel project," Zhao Shijie, chief engineer of the Dianzhong water diversion project in Yunnan, told the Global Times.

He told the paper that "rumours" about the tunnel were not new, but were "baseless".

The Yunnan tunnel project, which has no connection to the reported Tibet plan, began in August and the 660 km tunnel will divert water from the Jinsha river to cities in Yunnan, including capital Kunming.

The plan is estimated to cost 78 billion Yuan.

The Global Times said citing a 2006 media report that "a plan to divert water from Tibet to the northern parts of China was heatedly discussed in the 1990s. Over the decade, 208 lawmakers and 118 political advisers raised proposals and motions on the plan."

"However, the dream of massive water diversions has never been approved due to concerns of the huge cost and potential for damaging the landscape," it added.

Mei Xinyu, a scholar at the Chinese Academy of International Trade and Economic Cooperation, told the paper such a diversion would have unwanted ramifications. "I firmly oppose the project, as Xinjiang cannot afford this project. The estimated cost of diverting water from Tibet to Xinjiang would be five times that of Xinjiang's annual GDP. It may depend massively on central government subsidies and the assistance of local governments in other regions, which likely would lead to social instability."

The SCMP report quoted a researcher as saying "In five to ten years from now, the technology will be ready and the cost affordable, and the temptation of the benefits will be difficult to resist".

Another researcher, however, told the paper the Tibet tunnel would cost more than 1 trillion Yuan-five times the cost of the Three Gorges dam-which would be prohibitive.

A diversion of the Brahmaputra's waters would have ramifications for India and Bangladesh.

China's dams in Xinjiang and in Yunnan have caused concern in neighbouring countries, from Kazakhstan to Laos and Thailand.

While its dams on the Brahmaputra are at an early stage, India and China have established a working group mechanism on cross-border resources, but this has had a limited mandate so far. In 2013, both sides agreed to allow Indian hydrological experts to travel to Tibet to monitor flows of the Brahmaputra. China also agreed that year to provide hydrological data during the flood season from May until October every year.

That cooperation worked from 2014 to 2016, but data wasn't submitted this year. Chinese officials said it was because of upgrading monitoring stations.

China has so far built one 510 MW dam at Zangmu on the river's upper reaches, and has begun work on three more dams on the river. Beijing says these are run of the river dams for hydropower generation and won't store large volumes of water, but some experts have said it could alter the river's ecosystem.

The Brahmaputra isn't entirely dependent on flows from China, with a large catchment area in Arunachal Pradesh in India. India has commissioned a study to ascertain the percentage of flows dependent on China, with varying estimates from experts.