US and Iranian warships shadow each other as they ply the Gulf in a standoff over Iran's nuclear programme. But it is the small Iranian boats that worry the US Navy most, said experts.

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

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

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 US aircraft carrier, eventually turned around.

The encounter involving US 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.

'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 US Navy tries to avoid 'escalation of hostilities or miscalculations', as a result of their crossing, US 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 wide at its narrowest point, and as it sails through the Gulf, the aircraft carrier comes within range of the Iranian coastal missile defence system.

Over a month ago Iran warned another US 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.

Military experts say the US Navy's Fifth Fleet patrolling the Gulf - which always has at least one giant super carrier accompanied by scores of jets and a fleet of frigates and destroyers - is overwhelmingly more powerful than Iran's navy.

But it is the small boats that worry the US Navy most. Vice Admiral Fox said last week that Iran had built up its naval forces in the Gulf and prepared boats that could be used in suicide attacks.

Iran is thought to have increased the number of small boats based in the strait and around its Gulf Islands, and some boats are capable of carrying cruise missiles and rockets.

Five thousand sailors live on board the 20-storey USS Abraham Lincoln. Fifteen to twenty thousand meals are prepared daily with 800 pounds of vegetables, 900 pounds of fruit and 620 pounds of hamburger consumed every day.

For many of the sailors, Iran's threats were not always something they paid attention to. They often saw their mission in simpler terms.

'We want that oil to go where it needs to go in this world. We want people in this region to be able to get the products they can buy from Europe, from America, other regions of the world,' said Naval Aviator Matt Driskill, 33, who recently flew fighter planes over Libya and over Iraq in 2004.

The day after the passage, the mood on board the USS Abraham Lincoln is considerably more relaxed onboard.

Fighter aircraft that have been sitting on deck over the past 24 hours with their noses pointing towards Iran and readiness to launch within 15 minutes have now been stowed.

The admiral said the same preparations are taken every time the fleet passes a narrow canal. But he admitted that the fleet can be seen as show of force.

'Part of the reasons we exist is to be present around the world and be visible, even if itself the carrier, the planes and everything else on board is a show of force,' Shoemaker said.

If Iran attacks, the United States is ready to defend, Shoemaker said.

'It is certainly a possibility that they could take some actions to try and close the straits but we are prepared for that, we have the capabilities resonant in this force, in this strike group to respond if that happens,' he added.-Reuters