Finance leaders of the United States' closest allies lashed out at Washington's tariffs on steel and aluminum as they ended a three-day meeting on Saturday.

U.S. Treasury Secretary Steven Mnuchin was pressed by his Group of Seven counterparts to relay their "unanimous concern and disappointment" to President Donald Trump over the 25 percent steel and 10 percent aluminum tariffs imposed on major trading partners, including Canada, the European Union and Japan.

Finance ministers and central bank governors "had a frank exchange on the benefits of an open rules-based trading system and many highlighted the negative impact of unilateral trade actions by the United States," they said in a summary statement written by Canada.

They agreed at the meeting in the Canadian ski resort of Whistler that the issue will be discussed at the G-7 summit to be held in Charlevoix from Friday to Saturday where "decisive action is needed."

(Canadian Finance Minister Bill Morneau)

While Washington looked increasingly isolated by its allies, Trump defended his decision. As the meeting was wrapping up, he tweeted, "When you're almost 800 Billion Dollars a year down on Trade, you can't lose a Trade War! The U.S. has been ripped off by other countries for years on Trade, time to get smart!"

The G-7 comprises Britain, Canada, France, Germany, Italy, Japan and the United States. All six of Washington's partners in the group are now paying the tariffs, which were announced in March, mainly targeting China.

Mnuchin denied that the meeting had devolved into a "G-6 plus one," but his counterparts underscored the strong pushback against the Trump administration's protectionist stance.

"The Americans have decided to, in our mind, take an action that is not at all constructive. It's actually destructive to our ability to get things done," Canadian Finance Minister Bill Morneau said at a post-meeting press conference.

Japanese Finance Minister Taro Aso said it was "unprecedented" for the group to stand united against U.S. policy, while Bank of Japan Governor Haruhiko Kuroda said the effects of protectionist policies on the global economy "warrant attention."

The G-7 finance leaders agreed that the global economy is strengthening and that the expansion is likely to continue, though there are risks such as potential vulnerability in emerging market economies, which have seen their currencies sharply depreciate as a knock-on effect of interest rate hikes by the U.S. Federal Reserve.

They also agreed on the need for international cooperation in regulating crypto-assets, which are playing an increasingly large role in the financial system but may also be used for illicit transactions.