"It could be in December, it could be in January, it depends on how long the legislative process takes," EU foreign affairs spokesman Michael Mann told reporters in Brussels.
The United States and the EU have separate sanctions on Iran. Easing European restrictions would affect numerous areas including trade in petrochemicals, gold and other precious metals, financial transfers to purchase food and medicine, and the ability of third countries to use EU-based firms to insure shipments of Iranian oil again.
Mann said work on amending the EU regulations was already beginning, but cautioned that changes depend on the Iranian government living up to its end of the deal.
"It's important that both sides of the bargain are implementing this agreement, so we would coordinate timing-wise also with the Iranian side," the EU spokesman said.
Britain's Foreign Secretary William Hague put the total estimated value of sanctions relief at $7 billion over a 6-month period, but stressed it would not all come at once.
"They do not receive 7 billion on the first day and then decide if they want to implement their side of the agreement," Hague said, calling the amount of sanctions relief "a very small proportion" of the total frozen assets and value of sanctions applied to Iran.
"The way we're doing sanctions relief leaves Iran with a huge incentive" to go for a comprehensive agreement since Tehran wants complete sanctions relief, Hague said.
The deal reached Sunday will allow Iran to keep the central elements of its uranium program, while stopping its enrichment at a level lower than what is needed for nuclear arms. In addition to a six-month window for Iran to allow more U.N. access to nuclear sites, sanctions will be eased — notably in the oil, automotive and aviation industries — though not ended.
The agreement is a first step — one that Israel has condemned as a "historic mistake" that effectively accepts Iran as a threshold nuclear weapons state. Israel has found common cause with Saudi Arabia, which shares concerns about a nuclear-armed Iran and Tehran's growing regional influence.
On his return to Tehran, Iranian Foreign Minister Mohammed Javad Zarif told state television that the country was prepared for quick follow-up negotiations to keep the deal on track.
"We are ready to begin the final stage of nuclear agreement from tomorrow," said Zarif, who was greeted by hundreds of cheering students.
Many Iranians appeared upbeat about the deal and the possibility of an eventual end to sanctions, such as blocks on access to international banking networks that have crippled businesses and made once-routine transactions — such as paying tuition for a student abroad — a complicated process.
But hardline groups remained highly wary of any close cooperation with Washington.
An editorial in the conservative daily Kayhan described the U.S. as a deceitful power that could renege on its pledges even if Iran sticks with its part of the deal.
"The U.S. was not trustworthy. The Geneva deal lasted only one hour," it said in its front-page headline, referring to U.S. Secretary of State John Kerry's comments that there was no recognition of Iran's "right" to enrich uranium.