Japan imposed restrictions on exports used by South Korea’s chip and smartphone companies on Monday, ramping up tensions between the two countries over the use of forced labor during World War II.

The measures, which take effect from July 4, will significantly slow the export of several key substances used by South Korea’s chip and smartphone giants.

Seoul called the measures unfair and in violation of international laws, adding that it would respond firmly, Reuters reports.

“This export control measure by Japan is a measure that is banned in principle according to WTO agreements,” Park Tae-sung, a senior South Korean Trade Ministry official, said on Monday.

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