TEHRAN: Iran warned Britain's next prime minister Boris Johnson on Tuesday that it will "protect" waters of the oil-rich Gulf, amid a standoff between the two countries over the seizure of tankers.

In the face of rising hostilities with the United States, Iran's Islamic Revolutionary Guard Corps on Friday impounded a tanker sailing under the flag of US ally Britain.

The seizure of the Stena Impero has been seen as a tit-for-tat move after British authorities detained an Iranian tanker on July 4 in the Mediterranean on suspicion it was shipping oil to Syria in breach of EU sanctions.

"I congratulate my former counterpart, @BorisJohnson on becoming UK PM," Iranian Foreign Minister Mohammad Javad Zarif tweeted after Johnson beat his rival, Foreign Secretary Jeremy Hunt, in a vote by party members.

"Iran does not seek confrontation. But we have 1500 miles of Persian Gulf coastline. These are our waters & we will protect them."

Iran has impounded the Stena Impero at its port of Bandar Abbas for allegedly breaking "international maritime rules".

In new footage aired by Iranian state television, the crew of 18 Indians, three Russians, a Latvian and a Filipino are seen sitting around a table and seemingly going about their daily routines.

"Throughout history, Iran has been and will be the main guardian of security and free navigation" in the Gulf, President Hassan Rouhani said late Monday, adding that Tehran was not seeking to stoke tensions.

The head of Iran's navy, Rear Admiral Hossein Khanzadi, said in an interview published Tuesday that his forces use drones to closely observe "all enemy ships" going through the Gulf, "especially America's".