Britain is sending another warship to the Persian Gulf to defend freedom of navigation, the defence minister said on Saturday, after Iran seized a British-flagged tanker in the region.

Tensions have risen between Iran and Britain after Iranian commandos seized a British-flagged tanker in the Strait of Hormuz last month. That came after British forces captured an Iranian oil tanker near Gibraltar, accusing it of violating sanctions on Syria.

The navy said HMS Duncan, a type 45 frigate, sailed from Britain on Aug. 12, alongside HMS Kent, which was also heading to the Gulf to replace the HMS Duncan.

“Wherever the red ensign flies around the world, the U.K. stands by to protect freedom of navigation whenever is it tested,” defence minister Ben Wallace said.