UPDATE 1-Xi says BRICS should resolutely reject unilateralism

* Trump tariff threats give BRICS fresh impetus

* BRICS summit first since Trump warned of tariffs

* Xi says no winners in global trade war (Recasts with Xi and Ramaphosa opening remarks)

JOHANNESBURG, July 25 (Reuters) - Chinese President Xi Jinping said a global trade war should be rejected as it would leave no winner, calling on the BRICS nations on Wednesday to reject unilateralism in the wake of tariff threats by U.S. President Donald Trump.

Trump's warnings have given Brazil, Russia, India, China and South Africa fresh impetus to enhance trade cooperation, and officials at a three-day summit that kicked off in Johannesburg found a collective voice championing global trade.

The meeting of presidents from the trade bloc is the first since Trump's administration launched a push to rebalance trade multilateralism that Trump has deemed unfair, relationships that the United States once championed.

"We should be resolute in rejecting unilateralism," Xi said at the opening ceremony of the meeting of leaders from Brazil, Russia, India, China and South Africa.

"Unilateralism and protectionism are mounting, dealing a severe blow to multilateralism," he said. "China will continue to develop itself with its door wide open."

Xi also said the collective rise of emerging markets and developing countries "is unstoppable and will make global development more balanced." He urged the BRICS governments to observe international rules, regardless of their size. (Writing by James Macharia; Editing by Kevin Liffey)