A controversial law on safeguarding Iran's nuclear rights was found to be constitutional Wednesday despite opposition from the government, which considers it an obstacle in negotiations with world powers.

"The bill obliging the government to preserve nuclear rights and achievements... was not considered to be contradictory to religion nor the constitution and was approved by a majority of votes of Guardian Council members," council spokesman Nejatollah Ebrahimian said, quoted by Fars news agency.

Parliament adopted the law Tuesday.

The move came just one week before the deadline for an agreement on the disputed nuclear programme with the P5+1 -- Britain, China, France Russia and the United States plus Germany.

It exposed persistent tension between President Hassan Rouhani's government and lawmakers in Tehran, where hardliners routinely voice doubt about the merits of talking to the West.

Rouhani, a moderate who aims to end Iran's diplomatic isolation, wants an agreement that can lift sanctions that have hobbled the economy.

However, critics of his nuclear policy, including members of the conservative-dominated legislature, say too many concessions have been made and, using the bill, they demanded a bigger say.

The law says the government must "preserve the country's nuclear rights and achievements," a reference to retaining the ability to enrich uranium and keeping all nuclear facilities open.

Such demands have already been enshrined in an outline agreement struck on April 2 between Iran and the P5+1 powers.

Kerry heading to Vienna for final push in Iran nuclear talks
Washington (AFP) June 24, 2015 - US Secretary of State John Kerry will travel to Vienna on Friday for what is set to be a final push to seal historic, marathon talks to thwart Iran's nuclear ambitions.

The announcement fires the starting gun on what is meant to be the last phase of a gruelling diplomatic mission which has seen Kerry and his Iranian counterpart Mohammad Javad Zarif criss-cross the world to negotiate a ground-breaking deal.

Kerry, who is still walking on crutches after breaking his leg late last month, "will travel to Vienna, Austria, on June 26 to participate in the ongoing EU-coordinated P5+1 nuclear negotiations with Iran," his spokesman John Kirby said in a statement.

Iran and the six world powers negotiating the deal known as the P5+1 -- Britain, China, France, Germany, Russia and the United States - have set themselves a June 30 deadline to nail down one of the most complicated nuclear non-proliferation treaties ever achieved.

It will end a 12-year standoff between the Islamic republic and the West, which has long accused Iran of seeking to develop a nuclear bomb.

Tehran has denied the charge, insisting its atomic program is for civilian energy purposes only.

"There is the possibility that we can finish this by the deadline or a few days after the deadline," Zarif said earlier this week as he met his British, French and German counterparts in Luxembourg.

In April, Iran and the P5+1 agreed the main outlines of the deal after a bruising rollercoaster round of talks in Lausanne, Switzerland.

After two missed deadlines in July and then November last year, this built on an interim deal struck in Geneva in November 2013 after Iranian President Hassan Rouhani was elected.

According to the Lausanne framework, Iran will downsize its nuclear activities, slashing the number of centrifuges enriching uranium, which can be used in nuclear power but also when highly purified for a bomb.

It return Iran wants the lifting of a crippling network of US, EU and UN sanctions which have damaged its economy and barred it from world oil markets.

The powers hope the deal will ensure Iran would need at least a year -- compared with a few months in 2013 -- to produce a bomb's worth of enriched uranium material. Tight UN inspections would give ample notice of any such "breakout."

But Iran's supreme leader Ayatollah Ali Khamenei on Tuesday restated his red lines for a deal including the immediate lifting of US and UN sanctions on banking and the economy.

And he refused a key international demand that UN inspectors be given access to military sites to check whether they had been used for the possible development of a nuclear military program.

State Department spokesman Kirby has insisted several times this week that "the IAEA will and must have the access it needs to verify" the totality of Iran's nuclear program, referring to the International Atomic Energy Agency.