BEIJING (Reuters) - Myanmar's transition to democracy will not change the country's traditional friendship with China, Myanmar President Thein Sein was cited as telling Chinese Vice President Xi Jinping, amid concerns in China its neighbour could become a U.S. ally.

Chinese officials and media have expressed concern Washington's renewed interest in slowly democratizing Myanmar, formerly known as Burma, could be part of U.S. designs to dilute China's influence there and encircle China with pro-U.S. states.

Fears about China's influence in Myanmar have been bolstered not only by Washington's engagement with the country but also the U.S. military's strategic "pivot" back to Asia.

But during a meeting on the sidelines of a trade fair in southern China, Thein Sein said Beijing should not worry.

"Myanmar is at present in a transitional phase, but Myanmar pays great attention to developing relations with China, and its policy of seeing China has a true friend has not changed," China's foreign ministry cited Thein Sein as telling Xi.

"China has for a long time provided a large amount of sincere support and help, and stood at Myanmar's side at the most difficult of times. Myanmar's people will never forget this," Thein Sein added, in the statement released late Friday.

It is Thein Sein's second trip to China since he took office in March 2011. He goes to the United States after completing this trip.

With sanctions long blocking Western investments, China has emerged as Myanmar's biggest ally, investing in infrastructure, hydropower dams and twin oil-and-gas pipelines to help feed southern China's growing energy needs.

The United States, along with the European Union, Japan and other Western countries, have moved to ease sanctions on Myanmar following the new army-backed civilian government's efforts at pushing ahead with democratic reforms.

China has long worried about its ties with Myanmar, with a history of resentment of China among the Burmese population and fierce public opposition to a $3.6 billion Chinese-built dam at Myitsone that prompted Thein Sein to shelve the project last year, a move that stunned Beijing.

Xi did not directly address that issue, saying only that "both sides should work hard to guarantee the smooth progress of certain important cooperative projects".

A more crucial scheme - twin oil and gas pipelines being built at huge expense across Myanmar and into China - appears safe despite unhappiness among some residents who live along its route and conflict with ethnic minority rebels close to the Chinese border.

(Reporting by Ben Blanchard; Editing by Ron Popeski)