HANOI, March 23 (Reuters) - A push by the United States to upgrade ties with Vietnam this year is facing resistance in Hanoi, over what experts say are concerns that China could see the move as hostile at a time of tension between superpowers Beijing and Washington.

The United States is hoping for an upgrade in the relations this year, ideally to coincide with the 10th anniversary in July of its comprehensive partnership with Vietnam.

The United States is a major investor in Vietnam and the largest ever U.S. business mission visited the country this week.

Though it is Vietnam's biggest export market, it is currently ranked as a third-tier diplomatic partner for Hanoi. Its top tier consists of China, Russia, India and South Korea are, while its second tier, which Washington wishes to join, includes European countries and Japan.

A formal ties upgrade this year "is not considered realistic anymore," said Florian Feyerabend, the representative in Vietnam for Germany's Konrad Adenauer Foundation.

Though the move would be largely symbolic, Vietnam's leaders are hesitating, fearing possible retaliation from China, according to experts, who cited discussions with Vietnamese officials.

"Given the intensifying China-U.S. competition and proximity between China and Vietnam, Hanoi may feel reluctant to formally upgrade its comprehensive partnership with Washington," said Bich Tran, adjunct fellow at the Washington's Center for Strategic and International Studies.

China is Vietnam's biggest trading partner and a vital source of imports for its manufacturing sector.

The two neighbours have a long history of conflict and mistrust and remain at odds over islands, features and resources in the South China Sea.

Asked whether Vietnam was ready to upgrade ties with the United States this year, a spokesperson for its foreign ministry on Thursday said that would happen "when the time is right", stressing the strong relations they already enjoy.

High-level meetings could offer a chance for a last-minute breakthrough on the U.S.-Vietnam ties, the experts said, with diplomats hoping to arrange a meeting of their foreign ministers, while their leaders could meet on the sidelines of the G7 summit in Japan in May.

A spokesman for the U.S. embassy in Vietnam said the two countries were working together to elevate their partnership.

Le Hong Hiep, a Senior Fellow at Singapore's ISEAS–Yusof Ishak Institute, said there was no doubt Vietnam wants to upgrade ties with Washington, but it was unlikely to agree to that this year.

But "the upgrade may no longer be a priority for the U.S. in the future," he said.