Trump has given the European signatories of the deal a May 12 deadline to "fix the terrible flaws" of the 2015 nuclear agreement, or he will refuse to extend US sanctions relief on Iran.
Maas said that in upcoming meetings in Washington, German Chancellor Angela Merkel and French President Emmanuel Macron would urge Trump to stay in the deal. The agreement offered Tehran relief from sanctions in exchange for curbing its nuclear program.
"We believe it is extremely important to uphold this agreement. Were it to fail or the US to drop out, we would not have anything comparable to it and we fear that the situation would significantly deteriorate with everything that goes with it," he told reporters.
Maas, speaking on the sidelines of a meeting of foreign ministers from the Group of Seven leading industrialized nations, also said the G7 would formally call on Russia to contribute to solving the crisis in Syria.
Asked what the final communique from the two-day meeting would say, Maas told reporters that "It establishes again that there will be no political solution in Syria without Russia ... and that Russia has to contribute its share to such a solution."
'ENCOURAGE' TRUMP TO STAY
Iran has said it will stick to the accord as long as the other parties respect it, but will "shred" the deal if Washington pulls out. Iranian Foreign Minister Mohammad Javad Zarif called on European leaders on Monday to support it.
"It is either all or nothing. European leaders should encourage Trump not just to stay in the nuclear deal, but more important to begin implementing his part of the bargain in good faith," Zarif wrote on his Twitter account.
At a nuclear non-proliferation conference in Geneva on Monday there were repeated calls for parties to the deal to ensure its implementation and preservation.
UN High representative for Disarmament Affairs Izumi Nakamitsu said it "continues to be the best way to ensure the exclusively peaceful nature of Iran's nuclear program and to realize the promised tangible economic benefits for the Iranian people."
On a visit to Beijing, Russian Foreign Minister Sergei Lavrov said he had agreed with his Chinese counterpart to block any US attempt to sabotage the deal.
Trump's threat to restore US economic sanctions on oil-producing Iran have been one factor helping drive up global oil prices this month to their highest since late 2014.