US President Barack Obama left Washington for Paris Sunday to take part in a United Nations climate summit aimed at reaching a deal to curb global warming.
Air Force One, the presidential plane, took off late morning from Joint Base Andrews near the US capital.
Obama -- who has made climate change a priority for both his terms -- was at the 2009 Copenhagen summit, when an attempt to forge a global deal foundered upon divisions between rich and developing countries.
This time around, he is hoping for a more ambitious outcome.
"What makes this gathering different is that more than 180 nations have already submitted plans to reduce the harmful emissions that help cause climate change," Obama said in a Facebook post ahead of his departure.
"And America's leadership is helping to drive this progress."
In the aftermath of the November 13 terror attacks in Paris, Obama was the first to confirm that he would go ahead participate in the high-level gathering alongside some 150 other leaders, despite security fears.
"It's an opportunity to stand in solidarity with our oldest ally, just two weeks removed from the barbaric attacks there, and reaffirm our commitment to protect our people and our way of life from terrorist threats," Obama said in the post.
"It's also an opportunity for the world to stand as one and show that we will not be deterred from building a better future for our children."
Obama is due to spend two days in Paris over the course of which he has several bilateral meetings planned, including with Chinese President Xi Jinping and Indian Prime Minister Narendra Modi.
Work at the COP21 summit was due to start Sunday, with French Foreign Minister Laurent Fabius, who is presiding over the talks, convening a plenary session to set a roadmap for negotiations ahead of the official opening.
"I'm optimistic about what we can achieve - because I've already seen America take incredible strides these past seven years," Obama said.