Iranian President Hassan Rohani said on Monday the United States would regret a decision to leave Iran’s 2015 nuclear deal with world powers and Tehran would fiercely resist U.S. pressure to limit its influence in the Middle East. Rohani signaled Iran would be willing to remain in the deal as long as the EU, Russia and China gaurantee Iran's expectations are met.

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Germany and France on Monday vowed to stand by the 2015 nuclear deal between Iran and world powers even if the United States pulls out, with the German foreign minister saying the world would be less safe without it.

German Foreign Minister Heiko Maas said it was clear that the agreement made the world securer and there was a risk of escalation were it to be cancelled.

"We don't think there is any justifiable reason to pull out of this agreement and we continue to make the case for it to our American friends," Maas said during a joint news conference with visiting French Foreign Minister Jean-Yves Le Drian.

"We'll deal with the (U.S.) decision but like Jean-Yves said, we want to adhere to this agreement," Maas added.

Le Drian said France, Britain and Germany would keep to the 2015 nuclear deal with Iran irrespective of the United States' decision later this week because it is the best way to avoid nuclear proliferation.

"We are determined to save this deal because this accord safeguards against nuclear proliferation and is the right way to stop Iran getting a nuclear weapon," Le Drian said.

U.S. President Donald Trump, a long-time critic of the deal reached between Iran and six powers in 2015 before he took office, has threatened to pull out unless European signatories of the accord fix what he calls its “flaws” by May 12.

“If they want to make sure that we are not after a nuclear bomb, we have said repeatedly that we are not and we will not be ... but if they want to weaken Iran and limit its influence whether in the region or globally, Iran will fiercely resist,” Rohani said in a speech broadcast live on state television.

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Under the deal with the United States, France, Germany, Britain, Russia and China, Tehran restricted its uranium enrichment program to satisfy the powers that it could not be used to develop atomic bombs. In exchange, Iran received relief from sanctions, most of which were lifted in January 2016.

Foreign Minister Mohammad Javad Zarif warned that Tehran’s “fierce reaction to a violation of the nuclear deal with major powers will not be pleasant for America”, state TV reported.

Britain, France and Germany remain committed to the nuclear accord but, in an effort to keep Washington in it, want to open talks on Iran’s ballistic missile program, its nuclear activities beyond 2025 - when key provisions of the deal expire - and its role in the wars in Syria and Yemen.