Iran's supreme leader Ayatollah Ali Khamenei on Wednesday endorsed the country's nuclear deal with world powers but said "ambiguities" in the agreement must be guarded against.
Khamenei said he had approved the decision of Iran's top security committee, the Supreme National Security Council, to implement the deal but it must be "tightly controlled" and monitored "to prevent significant damage".
His remarks came in a letter to President Hassan Rouhani, a moderate who was elected in 2013 having promised to end the more than decade-long international dispute about Iran's nuclear programme.
Iran has always denied pursuing an atomic weapon but Western intelligence agencies alleged that work to develop a bomb had taken place.
In the letter, Khamenei, who has the final word on all policy decisions in the Islamic republic, said the July 14 agreement struck in Vienna had "many ambiguities and structural weaknesses".
"In the absence of tight control these could bring significant damage for the present and the future of the country," he said.
The deal, once implemented, will lift all nuclear-related international sanctions against Iran, in return for stringent curbs on its nuclear programme, including its enrichment of uranium.
The main provisions of the agreement, known as the Joint Comprehensive Plan of Action (JCPOA), extend for between eight to 15 years.
However, Khamenei said any new sanctions against Iran by the world powers (Britain, China, France, Russia and the United States plus Germany) would be a breach of the accord.
If this occurs "under any pretext by any country engaged in negotiations, it will be a violation of the JCPOA and the government will be obligated to stop the agreement," the 76-year-old leader said.
On Sunday, US President Barack Obama and the European Union announced measures to ensure the lifting of sanctions, including those on Iran's banks and energy sector, as soon as international monitors have closed a probe into possible military dimensions of Tehran's past nuclear activities.
The UN nuclear watchdog, the International Atomic Energy Agency (IAEA) is due to complete its final report into potential military aspects of Iran's nuclear programme by December 15.