The rules-based order must be defended, she said, and disputes could not be solved by nations taking "unilateral actions against their partners".
After leaving the summit Saturday, Trump announced that the USA was pulling back its endorsement of the G7 communique in part because of what he called "false statements" at a news conference by Canadian Prime Minister Justin Trudeau. This year's summit had a particular focus on trade.
"There were strong debate and disagreement on some issues, but after detailed discussions between both leaders and our teams we were able to find common ground and draw up a communique which reflected these discussions and the agreements we reached", May said at a parliament session.
Allowing Russia back into the G-7 is an unpopular idea among the other countries.
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Turning to Russia, Merkel said she could imagine Moscow re-joining the G7 format at some point, but there first had to be progress in the implementation of the peace plan for Ukraine.
The picture is a key moment in a two-day meeting that has gone viral.
"If they retaliate, they're making a mistake", Trump declared before departing the annual Group of Seven summit, which includes Britain, Italy, France, Germany and Japan.
The 28-member bloc - soon to lose the United Kingdom after the 2016 referendum to exit the European Union - managed to grapple with a financial meltdown and the biggest influx of refugees since World War II only with "great effort", Merkel said.
Senior Trump aides escalated the rhetoric on Sunday morning's news shows, with chief economic adviser Larry Kudlow calling Trudeau's remarks "a betrayal" on CNN's "State of the Union" and top trade adviser Peter Navarro saying on "Fox News Sunday" that "there's a special place in hell" for the Canadian leader.