Germany’s foreign minister Sigmar Gabriel said he was concerned over the US President’s stance on Iran’s international deal.
Mr Trump has threatened to “decertify” the 2015 agreement put in place to curb Tehran’s programme in exchange for lifting sanctions.
The President has called the Barack Obama-brokered deal, which was supported by Britain, Russia and China among others, an “embarrassment”.
But Mr Gabriel warned threats to end it could stop North Korea signing up to any similar deal, as fears continue the ongoing Pyongyang dispute could trigger World War 3.
Speaking in Berlin, Mr Gabriel said Germany was ready to increase diplomatic pressure on Iran and North Korea, but did “not want to see this agreement damaged”.
He said: “Our big concern is with, regard to North Korea, that it is very unlikely the North Korean dictatorship is ready to agree to an international agreement to renounce the building of nuclear weapons if the only agreement in the world that has allowed such a renunciation is at the same time called into question.”
Russia has also warned the threats to Iran could have “negative consequences”.
Russian President Vladimir Putin’s spokesman Dmitry Peskov said: “Obviously if one country leaves the deal, especially such a key country as the US, then that will have negative consequences.
“We can only try to predict the nature of these consequences, which we are doing now.”
Mr Trump is set to make a final decision on his Iran policy next week.
He is expected to stop short of pulling out of the agreement but give Congress 60 days to decide whether to reimpose sanctions.
Mr Trump said last week: “The Iranian regime supports terrorism and exports violence and chaos across the Middle East.
“That is why we must put an end to Iran’s continued aggression and nuclear ambitions.”
The President is also set to designate Iran’s most powerful security force, the Revolutionary Guards Corp, as a terrorist organisation.
Tehran has slammed the threat and promised a “firm, decisive and crushing” retaliation if it is followed through.
The latest criticism comes as both Moscow and Beijing urged restraint from Washington over Mr Trump’s weekend warning that “only one thing will work” against North Korea.
In recent weeks, North Korea has launched two missiles over Japan and conducted its sixth nuclear test, all in defiance of UN Security Council resolutions.
Mr Trump has lashed out at there hermit kingdom and last week dismissed the idea of talks as a waste of time.
He tweeted: “Our country has been unsuccessfully dealing with North Korea for 25 years, giving billions of dollars & getting nothing. Policy didn’t work!
And he raged on Saturday that “only one thing will work” against Pyongyang, implying military action was on the cards.