WASHINGTON  - The US military said on Wednesday that a new aircraft carrier strike group had arrived in the Arabian Sea and that another was on its way to the region, but denied any link to recent tensions with Iran and portrayed the movements as routine.

The shift in the powerful US naval assets comes at a moment of heightened tensions with Iran, which has threatened to close the Strait of Hormuz - the world's most important oil shipping lane - if US and EU sanctions over its nuclear program cut off its oil exports.

The US military has said it will halt any blockade of the strategic strait and the top US naval officer acknowledged on Tuesday that preparing for a potential conflict there was something that "keeps me awake at night."

Still, the Pentagon denied any direct link between recent tensions and the movement of aircraft carriers.  

"I don't want to leave anybody with the impression that we're somehow (speeding) two carriers over there because we're concerned about what happened, you know, today in Iran. It's just not the case," said Captain John Kirby, a Pentagon spokesman.

Military officials said the USS Carl Vinson arrived in the Arabian Sea on Monday to replace the outgoing USS John C. Stennis carrier strike group, which Iran last week warned not to return to the Gulf after departing in late December.

The Stennis was due to return to its home port in San Diego but the Pentagon did not say when that would happen.

Another carrier strike group, led by the aircraft carrier USS Abraham Lincoln, concluded a port visit to Thailand on Tuesday and was now in the Indian Ocean. It is on track to join the Vinson in the Central Command area of operations, which begins in the neighboring Arabian Sea.

It is "not unusual to have two carriers in the CENTCOM theater at the same time," a second US military official said.

Another official said there had been two carriers in the Gulf region at least twice in the past 18 months.

Tensions between Iran and the United States ratcheted up again in the past week. Iran started an underground uranium enrichment plant and sentenced an American to death for spying. Washington and Europe have stepped up efforts to cripple Iran's oil exports, and Tehran on Wednesday blamed US and Israeli agents for killing an Iranian nuclear scientist.

Israel declined to comment on the killing and the United States denied any US role and condemned the attack, in which the scientist was blown up by a bomb attached to his car by a motorbike hitman.  

Iran had warned the Stennis not to re-enter the Gulf and it is unclear when another US carrier will enter Gulf waters. The Pentagon has suggested only that, sooner or later, a carrier will pass through the Strait of Hormuz into the Gulf.

"We routinely operate our ships - all of our ships, all of our types of ships - inside the Arabian Gulf and that will continue," Kirby said.