The US Navy has said it will not tolerate disruption to a vital oil-trade route, following an Iranian threat to close it.
Iran warned it would shut the Strait of Hormuz if the West imposed more sanctions over its nuclear programme.
The US and its allies believe Iran is trying to develop a nuclear weapon - a charge Tehran denies.
Reacting to Iran's warning, a US Fifth Fleet spokeswoman said it was "always ready to counter malevolent actions".
The Strait of Hormuz links the Gulf - and the oil-producing states of Bahrain, Kuwait, Qatar, Saudi Arabia and the United Arab Emirates (UAE) - to the Indian Ocean. About 40% of the world's tanker-borne oil passes through it.
The US maintains a naval presence in the Gulf, largely to ensure the transport of oil remains open.
The strait "is not only important for security and stability in the region, but also is an economic lifeline for countries in the Gulf, including Iran", Pentagon spokesman George Little said.
"Raising the temperature on tensions in the Gulf is unhelpful", he said, but added that he was unaware of any hostile action directed against US vessels.
US Fifth Fleet spokeswoman Rebecca Rebarich told the BBC the navy would be ready to act if required: "The US Navy is a flexible, multi-capable force committed to regional security and stability, always ready to counter malevolent actions to ensure freedom of navigation."
Western nations recently imposed new sanctions against Tehran following a UN report that said Iran had carried out tests related to "development of a nuclear device".
Further measures being considered to target Iran's oil and financial sectors have brought a furious response from Tehran.
Vice-President Mohammad Reza Rahimi warned that "not a drop of oil will pass through the Strait of Hormuz" if sanctions are widened and Iran's navy chief Admiral Habibollah Sayari said that closing the strait would be "easy".
"The enemies will only drop their plots when we put them back in their place," Mr Rahimi was quoted as saying on Tuesday by the official news agency Irna.
Adm Sayari later told Iran's Press TV that closing the Strait of Hormuz would be "really easy" for Iran's armed forces "or, as Iranians say, easier than drinking a glass of water".
"But right now, we don't need to shut it as we have the Sea of Oman under control, and we can control the transit," he added.
Iran's threats to close the strait have not flustered markets and oil prices actually fell after a senior Saudi oil official said that Gulf Arab nations were ready to offset any loss of Iranian crude.
Earlier, US State Department deputy spokesman Mark Toner said the Iranian threats were simply "another attempt... to distract attention from the real issue, which is their continued non-compliance with their international nuclear obligations".
Iran's navy has been staging wargames in international waters to the east of the strait.
Adm Sayari said the manoeuvres were designed to show Gulf neighbours the power of Iran's military over the zone.
Washington and Israel have not ruled out military action against Iran's nuclear facilities if sanctions and diplomacy fail.
Iran has vowed to respond by attacking Israeli and US interests in the region.
An embargo on Iranian oil exports has been considered before but dismissed as it could also drive up global oil prices and harm Western economies, particularly in Europe.
It is believed the new measures could cut Tehran off from global energy markets without raising the price of fuel.