French Foreign Minister Laurent Fabius said the European Union could lift some sanctions against Iran next month, but that any easing will be "limited, targeted and reversible."
Fabius's comments came on Monday, a day after Iran agreed with the five permanent members of the U.N. Security Council and Germany on a deal to limit its enrichment activity and allow more inspections in exchange for some sanctions relief.
Iranian Foreign Minister Javad Zarif said on Sunday that his government will start implementing the steps it agreed to in the coming weeks, perhaps before the end of the year, but that all of the requirements cannot be fulfilled in one day.
Zarif also stressed the tenuous nature of the agreement, reached after months of talks between the two sides.
"All the measures that we will take, the confidence-building measures, are reversible, and they can be reversed fast. Of course, we hope we do not have to do this," said Zarif.
Diplomats, including U.S. Secretary of State John Kerry, are looking ahead to negotiations to come up with a comprehensive agreement on Iran's nuclear program, which the parties want in place within a year.
"Now, the really hard part begins, and that is the effort to get the comprehensive agreement which will require enormous steps in terms of verification, transparency and accountability. We know this, we have determined to work together, we will start today, literally, to continue the efforts out of Geneva and to press forward," said Kerry.
The text of Sunday's interim agreement includes sections outlining goals for a comprehensive deal. The steps include defining Iran's enrichment program "with practical limits," and ways to make its peaceful nature transparent. The agreement would also include the lifting of all U.N. sanctions, as well as those imposed by individual nations or smaller groups of nations.
The interim deal lasts for six months and calls for Iran to neutralize its stockpile of uranium enriched to 20 percent -- a level that is a short step away from weapons-grade. It does not prohibit Iran from continuing enrichment below the five percent level.
The agreement also calls for Iran not to make further advances in building a heavy water nuclear reactor in the city of Arak. Once operational, that facility could produce plutonium, another compound that can be used to make nuclear weapons.
In return for limiting enrichment, the United States, Britain, France, China, Russia and Germany agreed to temporarily lift some international sanctions that have weakened the Iranian economy.
The United States said Tehran will gain access to $4.2 billion in revenues from Iranian oil exports and $1.5 billion in proceeds from Iranian sales of precious metals, automobiles and petrochemicals.
Iranian leaders characterized the interim deal as a recognition of the country's nuclear rights and the beginning of an end to international sanctions.
Iranian President Hassan Rouhani said in a televised address Sunday that the deal recognizes what Iran claims is its right to enrich uranium.
Iran says its enrichment work is for peaceful purposes, but Israel and Western powers fear Iran could enrich its uranium to the high purity needed to develop nuclear weapons.
Kerry denied Iran's interpretation, saying the text "does not say Iran has a right to enrichment."
Israeli Prime Minister Benjamin Netanyahu denounced the deal, calling it a "historic mistake" and saying it marks the first time the international community has "formally consented" to Iran continuing enrichment.
White House officials say President Barack Obama telephoned Netanyahu Sunday, telling the Israeli prime minister that he wants the United States and Israel to start consultations immediately on efforts to negotiate a comprehensive agreement with Iran.
Obama also told Netanyahu that the U.S. remains firmly committed to Israel, which he noted has good reason to be skeptical about Iran's intentions.
Israeli leaders see a nuclear-armed Iran as a threat to their nation's existence because of Iran's frequent calls for Israel's demise.