U.S. President Donald Trump declared Thursday he was withdrawing the U.S. from the landmark Paris climate agreement, striking a major blow to worldwide efforts to combat climate change and distancing the country from many allies abroad.

He said the U.S. would try to re-enter an agreement but only if it can get more favourable terms.

Framing his decision as "a reassertion of America's sovereignty," he said, "I was elected to represent the citizens of Pittsburgh, not Paris."

Ending weeks of speculation, some of it fuelled by Trump himself and his cabinet members, he said, "As of today, the United States will cease all implementation of the non-binding Paris accord."

At a news conference in the White House Rose Garden on Thursday afternoon, Trump said the accord is "very unfair to the United States."

Introducing the president before the announcement, Vice-President Mike Pence said Trump is "choosing to put American jobs and American consumers first."

White House officials had signalled withdrawal was likely to reporters, but Trump has been known to change his mind at the last minute on major decisions, as happened recently with the NAFTA trade deal.

Abandoning the accord was one of Trump's principal campaign pledges, but America's allies have expressed alarm about the likely consequences.

The Paris agreement, a landmark 190-nation agreement to reduce earth-warming gases, was struck in 2015. Months later the U.S. and China jointly said they would sign off on the deal. The agreement went into effect for the U.S. in November 2016.

Jean-Claude Juncker, president of the European Commission, has criticized Trump's resistance to the deal, saying the U.S. president doesn't understand the fine print of the agreement and that the process of formally withdrawing could take two to three years.

Corporate leaders in America ranging from Coca-Cola to 3M to Dow Chemical and Procter & Gamble have expressed opposition to leaving the deal, saying it may lead to negative trade implications as it isolates the country from all of its traditional allies. The deal benefits U.S. manufacturing, and encourages investment in new technologies and opportunities, 30 leaders said in a public letter earlier this month.

Canada has said its participation in the deal is ironclad, regardless of the position the U.S. takes.

Trump was pressed at the recent G7 summit in Italy about this resistance to the deal, although Prime Minister Justin Trudeau said he wasn't prepared to "lecture another country on what they should do."

Leaders respond

Trudeau criticized the decision Thursday afternoon on Twitter.

"We are deeply disappointed," he said, adding that Canada is "unwavering in our commitment" to fight climate change.

Canada is unwavering in our commitment to fight climate change and support clean economic growth. —@JustinTrudeau

The Prime Minister's Office released a statement saying that Trudeau had spoken briefly to Trump following the announcement.

"The prime minister expressed his disappointment with the president's decision and also conveyed Canada's continued commitment to working internationally to address climate change," the statement said.

Environment Minister Catherine McKenna addressed the decision on Parliament Hill Thursday.

"Let's be clear, no one country can stop action on climate change, that the world is moving to a cleaner economy, that this is a huge economic opportunity," she said.

"It's unfortunate that the U.S. administration has said that they're pulling out of the Paris agreement, but you can't stop progress. I'm extremely proud that Canada stepped up and committed a historic $2.6 billion on climate change, and it's important that we support everyone to take serious action."

She tweeted Thursday morning that, regardless of the decision, "the world is going to keep marching on. The momentum is irreversible. And we only have one planet."

The world is going to keep marching on. The momentum is irreversible. And we only have one planet 🌎 <a href="https://t.co/j7bvNTDggS">https://t.co/j7bvNTDggS</a> —@cathmckenna

The office of the secretary general of the United Nations released a statement criticizing the move.

"The decision by the United States to withdraw from the Paris agreement on climate change is a major disappointment for global efforts to reduce greenhouse gas emissions and promote global security," Stéphane Dujarric, a spokesperson for Secretary General António Guterres, said in a written statement.

He added that "the transformation envisaged in the Paris agreement is already underway."

Former president Barack Obama said that Trump's decision on the Paris agreement reflects an 'absence of American leadership.' (Nam Y. Huh/Associated Press)

Former U.S. president Barack Obama said the Trump administration is joining "a small handful of nations that reject the future" by withdrawing from the Paris climate change pact.

Obama is defending the deal that his administration painstakingly negotiated. He said the countries that stay in the Paris deal will "reap the benefits in "jobs and industries created." Obama said the U.S. should be "at the front of the pack."

The former president said in a statement that Trump's decision reflects "the absence of American leadership." But Obama said he's confident nonetheless that U.S. cities, states and businesses will fill the void by taking the lead on protecting the climate.

Obama said that businesses have chosen "a low-carbon future" and are already investing heavily in renewable sources such as wind and solar.

'Weak position'

The David Suzuki Foundation also criticized the move.

"This top-down decision has left the U.S. administration stranded, spinning its wheels," said the foundation's science and policy director Ian Bruce. "By withdrawing from the agreement, the president has taken a weak position that will stall economic and environmental progress."

Tesla CEO Elon Musk said he will make good on his vow to depart the three White House economic advisory councils on which he sits now that Trump has announced the U.S. will withdraw from the accord.

"Climate change is real," said Musk. "Leaving Paris is not good for America or the world."