WASHINGTON (Reuters) -U.S. President-elect Donald Trump and Chinese President Xi Jinping discussed issues including TikTok, trade and Taiwan in a phone call on Friday, just days before Trump takes office again promising tariffs that could ratchet up tensions between the world's two biggest economies.

Both leaders were upbeat about the call, with Trump calling it "a very good one" and Xi saying he and Trump both hoped for a positive start to U.S.-China relations, according to Chinese state broadcaster CCTV.

It was the first phone call between the pair since Trump's election in November. There is an array of looming diplomatic and economic difficulties facing U.S.-China relations.

The U.S. Supreme Court on Friday upheld a law that mandates TikTok owner ByteDance divest TikTok's U.S. assets by Sunday to a non-Chinese buyer, or be banned on national security concerns.

"The call was a very good one for both China and the U.S.A. It is my expectation that we will solve many problems together, and starting immediately. We discussed balancing Trade, Fentanyl, TikTok, and many other subjects," Trump wrote in a post on his social media platform.

"President Xi and I will do everything possible to make the World more peaceful and safe!"

Xi raised China's concerns about Taiwan, which Beijing maintains is part of its territory, and said he hoped the U.S. will handle it with great care.

"The Taiwan issue concerns China's national sovereignty and territorial integrity, and he hopes the U.S. side will handle it with caution," according to CCTV.

Xi said the United States and China can have their differences but most respect each other's core interests, and that trade relations can be mutually beneficial without confrontation and conflict, comments similar to those he made during Trump's first term.

Trump offered strong support to Taiwan, including regularizing arms sales, in his first term. But during the campaign last year, Trump said Taiwan should pay the U.S. to be defended.

The Republican president-elect, who upended trade relations in his first term, is about to embark on an even more aggressive effort in his second term, pledging to impose 10% duties on all U.S. imports and 60% on goods from China.

Trump said on Jan. 6 that he and Xi have been communicating through representatives, expressing optimism about their relationship.