The three-day tour will include Hillary Clinton's first trip to Saudi Arabia
US Secretary of State Hillary Clinton has said the US would welcome peaceful engagement with Iran but not "while they are building their bomb".
She spoke at the US-Islamic World forum in Qatar during a trip to rally Arab support for tougher sanctions against Iran over its nuclear programme.
She said the West was "encouraging Iran to reconsider its dangerous policy decisions."
The three-day Gulf tour will include her first trip to Saudi Arabia.
The Obama administration is stepping up pressure on Iran by launching a diplomatic offensive in the Gulf.
Iran says its nuclear programme is to generate electricity so it can export more of its valuable oil and gas, but the West suspects it of trying to develop atomic weapons.
"The evidence is accumulating that that's exactly what they are trying to do," Mrs Clinton said.
"I would like to figure out a way to handle it in as peaceful an approach possible, and I certainly welcome any meaningful engagement, but... we don't want to be engaging while they are building their bomb."
Washington wants the UN Security Council to impose a fourth round of sanctions on Tehran.
Mrs Clinton is due to meet Turkish Prime Minister Recep Tayyip Erdogan, whose country opposes such sanctions.
Efforts to revive the Arab-Israeli peace process are also expected to be on the agenda during Mrs Clinton's tour.
The secretary of state delayed her trip by one day after her husband, former President Bill Clinton, underwent a heart procedure at a New York hospital.
In Qatar, she will hold talks with Prime Minister Sheikh Hamad bin Jasim al-Thani.
And in Saudi Arabia, Mrs Clinton will meet King Abdullah, as well as foreign minister Prince Saud al-Faisal.
A state department official travelling with Mrs Clinton said the US expected Saudi Arabia, which has growing trade relations with China, to persuade Beijing to abandon opposition to tougher sanctions against Iran.
"We would expect them [the Saudis] to use these visits, to use their relationships in ways that can help increase the pressure that Iran would feel," Jeffrey Feltman, assistant secretary of state, told reporters as Mrs Clinton began her trip.
Loss of revenue
China, which wields a veto on the UN Security Council, is still opposed to a fourth round of sanctions.
The BBC's Kim Ghattas, who is travelling with Mrs Clinton, says Beijing fears major loss of revenue from investments in Iran and disruption in oil supplies from the country.
The secretary of state is expected to press the Saudis to reassure the Chinese that the kingdom can offset any disruption.
Missile defence is also expected to be on the agenda - the US is beefing up the missile deterrent of its Arab allies to assure them that their security is key to Washington.
Mrs Clinton's two deputies will head to the region in the coming days, travelling to Israel, Jordan and Egypt.
And on Monday, William Burns, the under-secretary for political affairs, will travel to Lebanon and Syria.
Our correspondent says Washington is still hoping it can loosen the links between Damascus and Tehran.
She adds Lebanon currently holds a seat on the Security Council and its ties with Iran may prevent it from backing new sanctions.