G7 leaders on Saturday wrapped up their annual summit with an unprecedented display of division over climate change as U.S. President Donald Trump rebuffed pressure to toe the collective line in the club of powerful democracies.

Trump tweeted that he would reveal his hand only this week as to whether he will keep the United States in the Paris accord, a global pact on curbing carbon emissions that he vowed to jettison when campaigning for the White House.

The Group of Seven leading economies, in an extraordinary summit statement, acknowledged that six members were committed to upholding the 2015 accord while the United States stood apart.

"The United States of America is in the process of reviewing its policies on climate change and on the Paris Agreement and thus is not in a position to join the consensus on these topics," the statement said in highly-unusual language.

Despite the discord, Trump concluded his first overseas trip on a typically note, telling U.S. service families at the Sigonella base in Sicily he had "hit a home run" shortly before taking off to fly back home.

Condemning this week's deadly terror attacks in Manchester and Egypt, Trump said such atrocities "steel our resolve". "Together civilized nations will crush the terrorists, block their funding, strip them of their territory and drive them out of this earth," he said.

A short time later, scuffles broke out among protesters near the ancient hilltop town of Taormina where the summit had taken place under high security.

German Chancellor Angela Merkel, who also crossed swords with Trump on free trade at the G7, complained that the U.S. president was keeping his colleagues in the dark.

"The whole discussion on the topic of climate was very difficult, not to say very unsatisfactory," she told reporters, labelling the G7 deadlock as "six against one."

On trade, the G7 vowed "to keep our markets open and to fight protectionism", but also to combat "unfair trade practices" and help those left behind by globalization, after Trump came to power vowing "America First".

After starting his first presidential trip abroad wreathed in smiles, Trump ended it with rebukes, upbraiding America's European partners over military spending, trade and global warming.

An enduring motif of the G7, which represents the lion's share of global economic output, has been to champion free trade. At last year's summit in Japan, leaders issued a lengthy communique in support of resisting protectionism, as well as helping refugees and fighting climate change. But that was then, when Barack Obama still occupied the White House. Today, his successor is defiant about defying the G7 line after accusing China, Germany and others of cheating in international trade.

Trump reportedly described the Germans as "bad, very bad" in their trade practices while visiting Brussels this past week. "We had very hard deliberations and discussions about trade but we found a reasonable solution," Merkel said, stressing the G7 statement's commitment to open markets.