By Agencies

US and old allies on collision course on many issues amid doubts common ground will be found before Canada summit ends.

Differences over trade tariff disputes and calls to re-admit Russia have overshadowed a summit attended by leaders from seven of the world's wealthiest nations in Canada.

US President Donald Trump was due to leave the Group of Seven (G7) meeting earlier than other leaders to head to Singapore for a much-anticipated summit with North Korea's leader Kim Jong-un on June 12.

In a lengthy press conference ahead of his departure on Saturday, Trump said he spoke to world leaders about trade policies and called for the elimination of tariffs, trade barriers or subsidies.

"The United States has been taken advantage of for decades and decades," Trump told reporters in the Quebec resort town of La Malbaie.

He said that he did not blame G7 leaders for the "unfair" trade deals, but previous US presidents.

The informal annual gathering of seven advanced economies is attended by leaders from the US, Germany, Japan, Britain, France, Italy and Canada.

Collision course

But even before the G7 summit began, the US and other participants were on a collision course on a number of issues. 

Ahead of his trip to Canada, Trump fired off a provocative proposition by calling for Russia to be readmitted into the informal annual gathering of seven advanced economies.

The suggestion was quickly shot down by his European allies, except for Italy.

But tensions had already been seething over trade after the Trump administration confirmed on May 31 it would apply additional tariffs on steel and aluminum imports from Canada, Mexico, and EU countries, ending a two-month exemption period.

In response, Canada, Mexico and the EU said they were putting in place their own retaliatory measures.

During his press conference on Saturday, Trump warned foreign countries not to retaliate against the US tariffs. "If they retaliate, they're making a mistake," he said.

Another source of disagreement has been the unilateral US withdrawal from a multinational nuclear deal with Iran.

Trump announced the move last month to the dismay of partners in Europe and beyond who were left scrambling to keep the landmark 2015 agreement in place.

G7 participants were also angered by Trump's stance on climate climate - a key issue on the group's agenda.

Trump withdrew the US from the Paris climate accord and his attendance of a G7 session on the issue was in question well into the last minute.

Earlier on Saturday, Trump arrived late for a breakfast G7 discussion on gender equality, with summit host, Canadian Prime Minister Justin Trudeau, kicking off things off without waiting for those he described as "stragglers" to arrive.