By Francesco Guarascio and Casey Hall

HANOI/SHANGHAI (Reuters) -Fast-fashion online retailer Shein is leasing a huge warehouse in Vietnam, two people familiar with the deal told Reuters, its first in the country, in a move that could reduce its exposure to unpredictable U.S.-China trade tensions.

Shein, which was founded in China and sells products including $5 bike shorts and $18 sundresses, has agreed to lease nearly 15 hectares of industrial land for a warehouse near Ho Chi Minh City, Vietnam's commercial and trading hub, the two sources said, declining to be identified because the information was not public.

The online retailer, which almost entirely relies on China-based suppliers to make garments for the United States and other markets, has been caught in the crosshairs of a tit-for-tat China-U.S. trade war that threatens to upend global supply chains, despite a recent de-escalation.

One of the sources and a third person said Shein had been looking to rent more storage space in Southern Vietnam in addition to the large warehouse - equivalent to about 26 football pitches - which would store clothing and apparel from contractors before export.

Reuters could not establish where products housed in the leased warehouse would come from.

The retailer has previously flagged plans to source some products from Turkey and Brazil, and Shein suppliers from its traditional production base in southern China have told Reuters they are losing orders to Vietnam as some Chinese manufacturers opened factories there.

Shein, which is seeking a London listing, did not respond to questions from Reuters about the leasing of the warehouse space. It had previously denied it was shifting production capacity out of China.

The area around Ho Chi Minh City hosts an international airport, Vietnam's largest port for imports from China and another port that handles most seaborne exports to the United States.

Under a U.S. threat of punitive tariffs, Vietnam is cracking down on some imports from China, which Washington has said have for long been illegally rerouted through Vietnam to the United States to avoid higher duties.

Reuters had no access to the details of the warehouse lease and could not establish whether Shein would be able to revise its plans should U.S.-China trade tensions de-escalate further, reducing the appeal of diversification overseas.

Given the ongoing instability of the situation, however, analysts say Shein has little choice but to reduce its reliance on China.

"It would be dangerous for them not to diversify," said Manish Kapoor, CEO and founder of e-commerce supply chain solutions firm Growth Catalyst Group.