People walk near the Great Hall of the People at Beijing's Tiananmen Square. .

China will replace the world's largest economy within two decades, but it may not become a superpower and acquire the stature of the United States, says a report from the US intelligence community.

"China alone will probably have the largest economy, surpassing that of the United States a few years before 2030," says the US National Intelligence Council (NIC) in a report titled Global Trends 2030: Alternative Worlds.

But it does not mean that China will emerge as a superpower in the mould of the United States to play a major role in the international arena by forging alliances to take on global issues, says the report.

"Being the largest economic power is important... it isn't necessarily the largest economic power that always is going to be the superpower," said Mathew Burrows, counsellor to the NIC.

Aimed to promote strategic thinking, the report which is published every four years, captures the tide of "megatrends" that predicts transformation across the world. The latest report coincides with the re-election of Barack Obama as president of the United States.

It also predicts that Asia will emerge as the next global power wielding more "overall power" by 2030 overtaking the combined power of the US and Europe in terms of the size of the population, gross domestic product (GDP) and military spending as well as in technology development.

The report unfurls a gloomy scenario for the developed nations with an ageing population coupled with slowing growth and decline in living standards.

"The economies of Europe, Japan, and Russia are likely to continue their slow relative declines."

It also highlights climate change, global warming, depletion of natural resources and increased competition for food and water as the scenario for the future decades as the number of people who can be counted as middle class swells three-fold to three billion in two decades.

The report identifies the economies of China, India and Brazil to generate more middle class citizens with better education and health as well as more wealth and power.