In their first call since November, Chinese leader Xi Jinping warned US president Donald Trump to be “prudent” about supplying arms to Taiwan, according to a readout of their call provided by China’s foreign ministry.

“President Xi emphasised that the Taiwan question is the most important issue in China-US relations,” the readout said. “China must safeguard its own sovereignty and territorial integrity, and will never allow Taiwan to be separated. The US must handle the issue of arms sales to Taiwan with prudence.”

Hours later, the self-ruled island’s president, Lai Ching-te, said ties with the US were “rock solid”.

“The Taiwan-US relationship is rock solid, and all cooperation projects will continue uninterrupted,” Taiwan’s president, Lai Ching-te, told reporters during a visit to textile merchants in western Taiwan on Thursday.

Taiwan is a self-ruled democracy that China claims as its own territory, to be annexed by force if necessary. Beijing prohibits all countries it has diplomatic relations with – including the US – from having formal ties with Taipei.

Still, while the US doesn’t officially recognise Taiwan as a country, it is the island’s strongest informal backer and arms supplier.

In December, the US state department announced its largest-ever arms sales package to Taiwan, valued at more than $11.1bn and including missiles, artillery systems and drones. The package is yet to be approved by Congress.

China reacted angrily to the proposed arms sales, conducting two days of military drills around the island in late December, for which it dispatched air, navy and missile units.

The arms sales are also facing pushback from Taiwan’s opposition KMT party and some of its population, along with a proposed increase of defense spending to 3.3% of Taiwan’s gross domestic product.

Taiwan’s opposition-controlled parliament has blocked Lai’s budget plan, including a $40bn special defense budget, proposing instead a much smaller defense spending plan.

Late on Wednesday, Trump had said the call with Xi – which covered topics including Taiwan’s future – was “excellent” and “thorough” in a post on Truth Social.

The call also covered Russia’s war in Ukraine, “the current situation with Iran” and China buying oil and gas from the US, Trump wrote in the post.

He added that he was looking forward to a trip to China in April that will be the first of his current term in office.

Trump also said China was considering buying 20m tons of US soya beans in the current season, up from 12m tons in the previous season.