Bush and White House national security adviser Condoleezza Rice made it clear on 2 August that Washington intends to intensify international pressure on Tehran for failing to cooperate fully with the IAEA -- the nuclear watchdog agency of the United Nations.
But whether European nations engaged in negotiations with Iran are prepared to go along with that hard line remains unclear, even after Tehran announced this week it would not honor a pledge it made with them to suspend some nuclear-related activities.
Speaking to reporters at the White House, Bush said the United States and the European Union's "big three" members are working together to ensure there is, quote, "full disclosure, full transparency of [Iran's] nuclear weapons programs."
"We are paying very close attention to Iran, and we have [been paying attention] ever since I've been in office here," Bush said. "We are working with our friends to keep the pressure on the mullahs to listen to the demands of the free world."
Later, in an interview on Fox television network, Rice intensified the U.S. rhetoric, saying the regime in Iran "has to be isolated in its bad behavior, not 'engaged.'"
She added that Washington is working with the Europeans on what she called "a very tough set of resolutions" demanding Iranian compliance on the nuclear issue.
U.S. Secretary of State Colin Powell recently warned Iran that its case is likely to be referred to the UN Security Council for failing to meet IAEA commitments. A Security Council resolution could lead to sanctions on Iran.
But asked whether France would go along with U.S. plans to increase pressure on Iran, Rice said Washington will "just have to keep working with the French and the British and the Germans to make certain" that they follow the American position.
Whether they will is far from clear.
Analyst Steven Everts of London's European Center for Reform says there is growing frustration in Europe over Iran's failure to live up to vows that it has made on its nuclear activities. But Everts says Europe is unlikely to accept purely negative sanctions against Iran, noting that decades of similar sanctions have sparked n-o change in countries like Cuba.
"The only thing the Americans are putting on the table are further pressure, isolation, and sanctions -- and possibly more down the road," Everts said. "Europeans say Iran's a complex place; different people want different things. It should be possible to construct some form of positive incentives as well, whereby you say to the Iranians: 'If you accept denuclearization and the verification of denuclearization, here's what you can get in return, also from the United States.'"
A study released in July by the Council on Foreign Relations, a New York policy institute, also urged the United States to offer more incentives and fewer punishments as it seeks to effect change in Iran.
But the tough talk from Washington on 2 August appeared to reject the recommendations of that study, which also called for more engagement with Iran.
The Bush administration's warnings came after Iran announced that it had resumed building nuclear centrifuges, which the United States says are intended to enrich uranium for use in nuclear weapons.
Iranian Foreign Minister Kamal Kharrazi told a news conference in Tehran on 31 July that Iran would respect a pledge -- made in October to Britain, France, and Germany -- to suspend all uranium-enrichment-related activities.
But he added that a separate deal on halting centrifuge-building would not be respected: "[Based on our agreements in October], we have accepted [suspending] uranium enrichment and we are continuing that uranium-enrichment suspension based on our definition -- meaning that we have not restarted enrichment. But we are not committed to our agreement in Brussels in February on halting building centrifuge parts, because the three big European countries have failed to meet their commitments toward us. We said we could resume making centrifuges. As previously announced, we have started building centrifuge parts at our factory after we took the decision."
Iran has signed the Nuclear Non-Proliferation Treaty, under which signatory countries vow to refrain from acquiring atomic weapons and can be punished by Security Council action for violations.
The oil-rich Persian Gulf state denies any interest in nuclear weapons, saying it needs enriched uranium for nuclear power stations to meet increasing domestic demand for electricity.