The US has said it will not hesitate to defend its interests and those of its allies as Iran continues missile tests.
Secretary of State Condoleezza Rice said the US had increased its security in the region and Iran should not be "confused" about US capabilities.
Iran has tested missiles, including one that could reach Israel, over the past two days.
Meanwhile French energy giant Total has said it will not invest in Iran because it is too politically risky.
Israel has responded to the missile tests by putting on display one of its aircraft that it says can spy on Iran.
The state-run Israel Aerospace Industries says it has equipped its Eitam aeroplane with sophisticated intelligence-gathering systems.
Speaking on a visit to Georgia, Ms Rice said: "We take very strongly our obligation to defend our allies and we intend to do that.
"In the Gulf area, the United States has enhanced its security capacity, its security presence and we are working closely with all our allies... to make [sure] they are capable of defending themselves."
She added that it was about time Iran "got on the right side of the international community".
'Gesture of defiance'
The Iranian military said on Thursday that its Revolutionary Guards had carried out further medium- and long-range missile tests overnight, following tests on Wednesday.
The tests included the first night test of the Shahab-3 missile, said to have a range of 2,000km (1,240 miles), along with shore-to-sea, surface-to-surface and sea-to-air missiles, state media reported. The BBC's Jon Leyne in Tehran says this appears another gesture of defiance following the international condemnation of Wednesday's tests. Media reports appeared to be delivered as a matter of pride.
Gen Hoseyn Salami, the Guards' air force commander, said the tests demonstrated Iran's "resolve and might against enemies who in recent weeks have threatened Iran with harsh language".
State media quoted him as saying: "Our hands are always on the trigger and our missiles are ready for launch."
Meanwhile, the head of Total, Christophe de Margerie, told the Financial Times the company's planned development of the huge South Pars gas field in southern Iran would not go ahead.
"Today we would be taking too much political risk to invest in Iran because people will say: 'Total will do anything for money'," he said.
Total was the last major Western energy group to have seriously considered investing in the country's huge gas reserves.
Analysts say the move will be a big blow to the Iranian energy industry - it means Iran is now unlikely to significantly increase its gas exports until late into the next decade.
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