Nerves were strained as an Iranian patrol boat approached the USS Abraham Lincoln at speed.

A helicopter escort hovered above the vessel in a warning not to get any closer, and the grey boat, tiny compared to the massive U.S. aircraft carrier, eventually turned around.

The encounter involving U.S. and Iranian boats, common in recent weeks, underscores rising tensions in the Gulf region between rival powers since Tehran threatened to close the Hormuz Strait, the world’s most important oil shipping waterway, over Western moves to ban Iranian crude exports.

U.S. and Iranian warships shadow each other as they ply the Gulf in a standoff over Iran’s nuclear program the West fears is aimed at producing an atomic weapon. Many fear any incident could trigger a war.

“I watch it morning, noon and night. I take it (the threat to close Hormuz) very seriously. In fact it’s pretty much my life these days,” the commander of U.S. naval forces in the Gulf region, Vice Admiral Mark Fox, told a news conference in Bahrain ahead of the fleet’s voyage earlier this week.

The fleet, known as “Carrier Strike Group Nine” has been making forays through Hormuz despite the Iranian threats.

The 10-hour voyage through the waterway on February 14 was the second time the fleet had been through Hormuz in two months. Passage is done on a need-only basis as the U.S. Navy tries to avoid “escalation of hostilities or miscalculations,” as a result of their crossing, U.S. officials say.

With four helicopters circling overhead and two destroyers leading, the carrier entered Hormuz while up in the watch tower, some seven Navy commanding officers, intelligence chiefs and legal experts were gathered in a small but busy control room.

They inspected the Gulf waters intently. The head of the fleet, Rear Admiral Troy Shoemaker, spotted two small boats, thought to be of smugglers, being battered by the high waves.

“It is going very well, relatively quiet. We have had a couple of surveillance aircraft, a helicopter and UAV (unmanned aerial vehicle) but nothing in the way of surface activity,” Shoemaker said, referring to activity from Iranian side.

The geography of the Strait, where a third of the world’s seaborne oil trade passes, is challenging for a fleet of this size.

The waterway is 34 km (21 miles) wide at its narrowest point, and as it sails through the Gulf, the aircraft carrier comes within range of the Iranian coastal missile defense system.

Over a month ago Iran warned another U.S. aircraft carrier, the USS John C. Stennis, not to return to the Gulf after it passed through the Strait. But that has not deterred the USS Abraham Lincoln.

“We routinely operate close to them while we operate in the Arabian Gulf,” Shoemaker said.

“They have ships that would come out and observe us as you would expect we would do in our territorial water back in the United States, so all those exchanges were very professional,” he added.

The Iranians make their presence felt every time U.S. forces cross the strait, by almost escorting the fleet either by air or using patrol boats. The U.S. in return reassesses the threat from Iran on regular basis by studying Iranian activity.

READ MORE: REUTERS