"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.
In the U.S., President Barack Obama defended the agreement, declaring that the United States "cannot close the door on diplomacy."
"Tough talk and bluster may be the easy thing to do politically, but it's not the right thing to do for our security," he said during an event in San Francisco.
But hardline groups in Iran remained highly wary of any close cooperation with Washington.