Officials in Malaysia have pledged more vigilance and tougher action against retail fuel operators caught selling subsidised gas to foreign-registered vehicles, according to local news reports.

Thousands of vehicles have crossed the border between Singapore and Malaysia since it was fully opened on April 1, ending a two-year, pandemic-forced closure.

Watch the video here.

The Johor division of the domestic trade and consumer affairs ministry said it would step up enforcement against operators caught selling the fuel to foreign-registered cars, with companies liable for fines of up to RM2 million.

Former prime minister Najib Razak also highlighted the issue in a Facebook post, calling for increased monitoring of such sales.

Malaysia previously banned the sale of RON95 petrol to cars not registered locally because that grade of petrol is subsidised for residents.

Singapore currently sells that petrol grade at more than four times what it costs in Malaysia.

The consumers’ desire for cheaper petrol comes as global oil prices surge.

Singapore had earlier made it mandatory for all locally registered vehicles to have at least three-quarters of their fuel tanks filled before driving out of the country.

“It’s pure price arbitrage that happens to be illegal, of course,” said Wellian Wiranto, an economist at Oversea-Chinese Banking Corp.

“The Malaysian government already has the legal framework in place to target such malfeasance, so it’s probably a matter of a tougher crackdown within that framework.”