WASHINGTON -- U.S. President Donald Trump insists that Iran will not acquire nuclear weapons, although he declined to speak about what measures he would use to deter Tehran should the landmark 2015 nuclear deal collapse.

“I don't talk about whether or not I would use military force," Trump said in a joint news conference with visiting German Chancellor Angela Merkel.

"But I can tell you this -- they will not be doing nuclear weapons. That I can tell you. OK? They are not going to be doing nuclear weapons. You can bank on it. "

Trump’s comments come ahead of a May 12 deadline he has set to fix what he calls “flaws” in the 2015 nuclear accord signed by Tehran and six world powers or face a U.S. withdrawal.

The deal provides Tehran with relief from sanctions in return for curbs on its nuclear program, but Trump has long said the terms are not strict enough.

Germany, France, Russia, China, and Britain have also signed the accord and have urged Washington to remain a part of the deal, saying it is the best way to prevent Iran from acquiring nuclear weapons.

Merkel said the accord is “anything but perfect," but she described it as "one building block” and a “first step” that has contributed to slowing down Tehran’s nuclear activities.

She added that her country will remain in close contact with U.S. authorities as the deadline approaches.

Merkel also said she and Trump discussed Russia's "illegitimate actions" regarding its annexation of Ukraine's Crimean Peninsula and its support for separatists in eastern Ukraine, saying they were working "very closely" together on the situation.

"The principle of territorial integrity of a country such as Ukraine is one that needs to be upheld and needs to be enforced," she said, without providing details of their talks.