Singapore will not negotiate with Iran over the passage of ships through the Strait of Hormuz or the payment of tolls, Deputy Prime Minister Gan Kim Yong said Thursday at Semafor World Economy.

Gan said any negotiations would implicitly undermine the bedrock principle of international law that ships must be allowed to navigate international waters unimpeded.

“We are not able to support any negotiation because this right is not negotiable,” Gan said. Responding to a question about the US blockade of the strait, he cited the UN Convention on the Law of the Sea, which ensures freedom of navigation.

The issue is a critical one for the Southeast Asian city-state of 6 million people. The Port of Singapore is the world’s second-business container port, and trade accounts for some three times the country’s GDP. Singapore also sits astride the Strait of Malacca, another major chokepoint for trade.