The U.S. on Saturday urged Britain to pull out of the 2015 Iran nuclear deal and instead join the U.S. to "[turn] up the pressure" on Iran.

U.S. ambassador to the United Kingdom, Robert Wood Johnson, wrote in the British paper the Sunday Telegraph, that "Iran grew bolder" after signing on to the Obama-era deal.

“It is clear that the danger from Iran did not diminish in the wake of the [2015 Iran] deal,” Johnson wrote. “Far from becoming a more responsible member of the international community, as we had all hoped, Iran grew bolder.”

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“It is time to move on from the flawed 2015 deal," he continued. "We are asking global Britain to use its considerable diplomatic power and influence and join us as we lead a concerted global effort toward a genuinely comprehensive agreement.”

Johnson's comments come days after the U.K.'s Middle East minister Alistair Burt said Britain would not join the U.S. in sanctioning Iran, Reuters reported.

Burt said U.K. remains open to discussions with the U.S. over addressing mutual concerns about Iran, but said the Iran nuclear deal is integral to Britain's regional security.

In a controversial move, President Trump Donald John TrumpSteele Dossier sub-source was subject of FBI counterintelligence probe Pelosi slams Trump executive order on pre-existing conditions: It 'isn't worth the paper it's signed on' Trump 'no longer angry' at Romney because of Supreme Court stance MORE pulled the U.S. out of the deal in May, breaking with longtime European allies. The move also upheld a campaign-era promise of Trump's and dealt a significant blow to former President Obama's international agreement.

This week, the Trump administration announced that it would reimpose sanctions on Iran that were lifted as part of its nuclear agreement with the country. The sanctions will take effect at 12:01 a.m. Tuesday.

The reimposition of sanctions comes amid protests across Iran, with demonstrators voicing dissatisfaction with a weak economy and financial corruption.

Tensions between Trump and Iranian President Hassan Rouhani have ratcheted up in recent months amid U.S.-imposed sanctions on Iran.

But administration officials say that increasing pressure on Tehran — including through sanctions — aligns with the possibility of talks with Iranian officials.