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DUBAI, July 21 (Reuters) - Iran's Supreme Leader Ayatollah Ali Khamenei on Saturday backed President Hassan Rouhani's suggestion that Iran may block Gulf oil exports if its own exports are stopped and said negotiations with the United States would be an "obvious mistake".

Rouhani's apparent threat earlier this month to disrupt oil shipments from neighbouring countries came in reaction to looming U.S. sanctions and efforts by Washington to force all countries to stop buying Iranian oil.

"(Khamenei) said remarks by the president ... that 'if Iran's oil is not exported, no regional country's oil will be exported,' were important remarks that reflect the policy and the approach of (Iran's) system," Khamenei's official website said.

Iranian officials have in the past threatened to block the Strait of Hormuz, a major oil shipping route, in retaliation for any hostile U.S. action.

Khamenei used a speech to foreign ministry officials on Saturday to reject any renewed talks with the United States after President Donald Trump's decision to withdraw from a 2015 international deal over Iran's nuclear programme.

"The word and even the signature of the Americans cannot be relied upon, so negotiations with America are of no avail," Khamenei said.

It would be an "obvious mistake" to negotiate with the United States as Washington was unreliable, Khamenei added, according to his website.

The endorsement by Khamenei, who has the last word on all major issues of state, is likely to discourage any open opposition to Rouhani's apparent threat.

Khamenei also voiced support for continued talks with Iran's European partners in the nuclear deal which are preparing a package of economic measures to offset the U.S. pullout from the accord.

"Negotiations with the Europeans should not be stopped, but we should not be just waiting for the European package, but instead we should follow up on necessary activities inside the country (against U.S. sanctions)," Khamenei said.

France said earlier this month that it was unlikely European powers would be able to put together an economic package for Iran that would salvage its nuclear deal before November.

Iran's oil exports could fall by as much as two-thirds by the end of the year because of new U.S. sanctions, putting oil markets under huge strain amid supply outages elsewhere in the world.

Washington initially planned to totally shut Iran out of global oil markets after Trump abandoned the deal that limited Iran's nuclear ambitions, demanding all other countries to stop buying its crude by November.

But it has since somewhat eased its stance, saying that it may grant sanction waivers to some allies that are particularly reliant on Iranian supplies. (Reporting by Dubai newsroom Editing by Andrew Heavens and Ros Russell)