South Africa’s ruling ANC party rebuked the opposition mayor of Pretoria for visiting Taiwan last month in what it called a violation of its foreign policy that recognises only the People’s Republic of China.

The ANC said any officials making trips at odds with South Africa’s foreign relationships should be stripped of their passports. China is South Africa’s biggest trading partner.

Africa’s part in the long-running global diplomatic row over Taiwan’s disputed independence from China drew new attention last month after the island nation of Sao Tome and Principe reversed its position and recognised only Beijing.

Relations between Africa and Beijing are important in part because China is a major investor in many African economies.

The mayor of Pretoria, Solly Msimanga, travelled to Taipei last month to woo investors, the country’s main opposition Democratic Alliance (DA) party, which governs the South African capital, said late on Monday.

The South African government, which was an ally of Taiwan during the era of white minority rule but which has recognised only Beijing since 1997, described the Msimanga’s visit as “highly regrettable”.

“(We were) made aware of the visit and advised Mayor Msimanga not to undertake it as it would constitute a breach of our One China Policy,” the foreign ministry said in a statement issued late on Monday.

The ANC called on the Foreign Ministry to “confiscate all official and diplomatic passports from officials who are found to be wilfully undermining our foreign policy”. The ANC also urged the ministry to issue a strong protest over Msimanga’s Taipei invitation to Taiwan’s liaison office in South Africa.

But the Democratic Alliance rejected pressure not to have contacts with Taiwan. A DA statement said that “neither the ANC nor the national government it runs can dictate who DA mayors meet with in order to obtain job-creating investment”.

The ANC suffered its worst election results since the 1994 end of apartheid in an August vote that swept DA candidates including Msimanga to power as mayors in three major cities.

U.S. President-elect Donald Trump put in question the “one-China” policy followed for decades by Washington and Beijing when he accepted a congratulatory phone call from Taiwan’s leader shortly after his Nov. 8 victory.

(Reporting by TJ Strydom; editing by Mark Heinrich)