Small Iranian military motorboats approached US vessels passing through the Strait of Hormuz twice last week but the Pentagon said the interactions were not seen as hostile, even at a moment of heightened tensions between the two countries.

Video released by the Pentagon showed the armed boats with the Iranian Revolutionary Guard Corps' navy approaching within several hundred meters of the USS New Orleans, an amphibious transport ship, on January 6, a US military official said.

The same day, a similar incident occurred with the US Coast Guard cutter Adak, with the Iranian boats seen riding in its wake, guns visible.

'This interaction between US naval vessels and the Iranian vessels is commonplace,' said Captain Jane Campbell, a Pentagon spokeswoman. 'There is nothing in these that shows any kind of hostile intent.'

US officials say it is routine to take video of such incidents and the US military decided to release imagery at the request of news organizations.

Nine American vessels have passed through the strait since the start of the year. It was not immediately clear whether any of the other seven had been approached in this manner by Iranian vessels.

The approaches come at a time of concern about the possibility of a clash between the United States and Iran in the Strait of Hormuz, the world's most important oil shipping lane.

Tehran has threatened to close the strait if new U.S. and EU sanctions over its nuclear program cut off Iranian oil exports. Iran has also threatened action if another U.S. carrier moves into the Gulf.

Defense Secretary Leon Panetta has warned that closure of the strategic waterway would be seen as a red line by the United States and would require a response.

The Pentagon also sought on Friday to discourage speculation the US military was quietly building up its forces in the region to counter any perceived threat.

The number of US forces in Kuwait has grown to about 15,000 in recent weeks, including two combat brigades, as troops have withdrawn from Iraq following the end of the war there.

Navy Captain John Kirby, a Pentagon spokesman, said he was not aware of any decision to permanently increase the number of troops based in Kuwait.

The US force there has expanded temporarily because a brigade deployed to Iraq at the end of the war had been shifted to Kuwait to finish its deployment, he said.

Force numbers in any given location shift regularly depending on needs, Kirby said.

'I'm not aware of any plus-up that's been ordered into Kuwait. And I don't think the numbers would bear out that there is, in fact, a huge plus-up in Kuwait,' he told reporters.

'Iran is certainly a factor in our discussions with our allies and in our thinking about the future of the Middle East - there's no question about it - thanks to their destabilizing behavior,' Kirby told reporters at the Pentagon.

'But I want to disabuse everybody of the notion that there's some kind of quiet increase going on, specifically aimed at some sort of contingency planning for any one country in that part of the world,' he said.

Earlier this week, the military said a second aircraft carrier had arrived in the Arabian Sea and a third was on its way to the region.

The Pentagon portrayed that as a normal rotation, with one ship en route to its home port. Kirby said it was not unusual to have two carriers in the region.

Defense Secretary Leon Panetta has said the United States would have about 40,000 troops in the region after the withdrawal of American forces from Iraq at the end of 2011. Kirby declined to specify where they would all be located.-Reuters