Iran's supreme leader gave approval Wednesday to the country's landmark nuclear deal with world powers, effectively silencing any internal opposition but warning that Tehran must keep pressure on the West to ease economic sanctions as promised.

The endorsement by Ayatollah Ali Khamenei marked the final and critical step to approve the accord, which had already been cleared by Iranian lawmakers and an important panel of ruling clerics.

Khamenei - who has the final say on all state matters - had tacitly supported the deal by allowing negotiators to hammer out provisions to limit Iran's nuclear program in exchange to lifting many nuclear-linked sanctions.

But Khamenei has tempered his views recently by insisting that the nuclear deal will not open channels for other talks with the United States and possible better ties between the two foes.

Khamenei's stamp of approval was contained in a letter to President Hassan Rouhani that was read on state television. It also signifies a major boost for Rouhani and his reform-minded political allies, who have been accused by Iranian hard-liners of giving away too much in the nuclear deal.

But Khamenei had his own caveats on the deal reached with the United States and five other nations. He said it "suffers from multiple structural weaknesses and ambiguous points" that require "precise and constant vigilance" from Iranian authorities, according to translations by the Associated Press.

More specifically, he warned the West that "any remarks saying the structure of sanctions will remain in place are considered a breach" of the agreement forged in July after years of talks.

Iran insists it does not seek nuclear arms but demands the ability to enrich uranium to make nuclear fuel for energy-producing reactors and medical applications. The Obama administration says the nuclear deal gives international inspectors sweeping access to make sure Tehran lives up to its pledges.

Earlier this week, United States and the European Union set down the legal groundwork for the eventual lifting of sanctions as part of the nuclear deal.

President Obama on Sunday signed an order directing government agencies to prepare to issue waivers on sanctions related to Iran's nuclear program. The European Union announced legal actions that clear the way for the release of an estimated $100 billion in frozen Iranian assets.

The measures will not take effect for months - after Tehran satisfactorily meets conditions on mothballing or converting some nuclear sites and equipment, reducing enriched-uranium stockpiles and addressing claims of past nuclear-related projects at military sites.