The White House said Tehran's compliance with the promise to open Iran's nuclear program to inspectors and work with the UN nuclear watchdog agency was a move in the right direction.
"It would be a positive step in the right direction. Full compliance by Iran will now be essential," spokesman Scott McClellan told reporters as US President George W. Bush made an overnight stop in Singapore on an official visit.
On Tuesday, yielding to international demands for it to prove it is not developing nuclear weapons, Iran agreed to allow tougher inspections of its nuclear sites and to halt uranium enrichment.
In Vienna, the top official with the International Atomic Energy Agency (IAEA) Mohamed ElBaradei said that, in response to Tehran's promise of full cooperation, the body expected a swift and "complete declaration of all its past nuclear activities".
Stretched by military engagement in Iraq and aiming to resolve the crisis over North Korea's nuclear weapons drive, Washington has said it would favor a diplomatic solution to the Iran nuclear issue.
The White House spokesman underlined that Iran would have to totally conform to demands by the IAEA to prove it was not seeking to make an atomic weapon.
The IAEA has set an October 31 deadline for Tehran to conform to a whole series of measures to prove its nuclear program is not bellicose.
On Tuesday, Iran agreed to give the foreign ministers of Germany, France and Britain guarantees by signing the additional Non-Proliferation Treaty, suspending all uranium enrichment activities and showing cooperation and "total transparence" to the nuclear watchdog agency.
At the US State Department, deputy spokesman Adam Ereli called the pledge "a first step and this is only a first step."
"We will be watching to see whether they do that."
He said Washington was most interested in compliance from Iran.
"The requirements of the IAEA and the requirements of the NPT (nuclear Non-Proliferation Treaty) are not renegotiable and that remains the case today," Ereli said.
He declined to comment on whether Tehran might agree to international cooperation on its civil nuclear program in response. "What steps after they fulfill the requirements are another issue. But this is a first step."
A senior US official, speaking on condition of anonymity, told reporters that "if Iran does everything it's supposed to do it will have the same rights and access to technology as any other country."
Ereli sought to play down the impression that the European approach to maintain dialogue with the Islamic republic had produced better results than Washington's hardline stance.
The United States broke off relations with Iran 23 years ago, and Bush included the country in his "axis of evil".
Ereli commended efforts of the British, French and German foreign ministries in gaining Tehran's agreement, and said: "We share the same goal."
He said that Secretary of State Colin Powell, who is in Kenya, had discussed the matter Tuesday with British Foreign Secretary Jack Straw.
"The three governments made an effort to keep us informed of their plans. This statement today is an indication of what can be achieved when we all work together to send the same firm message that Iran needs to fulfill its obligations," Ereli added.