US Vice-President Dick Cheney has issued a warning to Iran from an aircraft carrier in the Gulf.
Mr Cheney is calling on allies during his Middle East tour
He said Washington and its allies would keep Tehran both from restricting sea traffic and developing nuclear arms.
Mr Cheney, who is touring the Middle East, was speaking from the deck of the USS Stennis as the ship sailed about 150 miles (240 km) from Iran's coast.
He added that the presence of the American navy in the region sent a clear message to Iran.
"We'll keep the sea lanes open," Mr Cheney said.
"We'll stand with others to prevent Iran from gaining nuclear weapons and dominating this region."
The US accuses Iran of seeking to build nuclear weapons. Washington has refused to hold direct talks until Tehran halts its uranium enrichment.
President Ahmadinejad insists Iran has a right to nuclear technology
Iran says its nuclear programme is entirely peaceful and is solely aimed at producing civilian nuclear power.
Mr Cheney told the aircraft carrier's sailors and marines: "It's not easy to serve in this part of the world. It's a place of tension and many conflicts."
His remarks come a week after the Secretary of State Condoleezza Rice briefly met the Iranian foreign minister at an international conference about Iraq.
The US has had no formal ties with Iran since the 1979 Iranian revolution.
Mr Cheney is in the Middle East to ask allies to do more to help the Iraqi government and to curb Iran's growing power. On Saturday he is due to visit Saudi Arabia.
In March Iran seized 15 British sailors and Royal Marines patrolling Iraqi waters. Tehran said they had entered Iranian waters, but Britain denied this.
The 15 were freed after 13 days in captivity.