Speaking to United States network Fox News ahead of meetings in Washington with U.S. administration officials, the British Foreign Secretary said Trump was "right to see flaws" in the deal but "Plan B does not seem to be to me particularly well-developed at this stage".
US President Donald Trump has threatened to retreat from the deal by not extending sanctions waivers when they expire May 12, unless European signatories of the accord fix what he calls its "flaws".
Johnson spoke to "Fox & Friends" Monday before his talks with the Trump administration, acknowledging that Iran is "behaving badly" and the agreement's "flaws" must be fixed. "He set a challenge for the world".
"I will be announcing my decision on the Iran Deal tomorrow from the White House at 2:00pm (1800GMT)", Trump tweeted on Monday.
Mr Le Drian said: "We will continue it independently of the American decision".
Under the pact, global sanctions were eased in return for verifiable limits on Iran's nuclear program, but Iran says it is not reaping the rewards despite complying with the deal.
Boris Johnson, the British Foreign Secretary, has warned that an arms race could develop in the Middle East if the U.S. backs out of the Iranian nuclear agreement. "What if the Iranians do rush for a nuclear weapon?" he asked, raising the prospect of having to bomb the facilities.
"We see no alternatives to the control mechanisms on and the restrictions to the Iranian atomic programme". "The wisest course would be to improve the handcuffs rather than break them".