US President Barack Obama landed in Paris on Sunday night to take part in a United Nations summit aimed at sealing a deal avert catastrophic climate change.
US President Barack Obama landed in Paris on Sunday night to take part in a United Nations summit aimed at sealing a deal avert catastrophic climate change.
Presidential plane Air Force One arrived at the French capital's Orly airport at little before midnight (2300 GMT).
All eyes are on the 150 world leaders who will meet at the UN conference, and hundreds of thousands of people rallied around the world on Sunday to put pressure on negotiators to seal an ambitious deal.
"I'm optimistic about what we can achieve -- because I've already seen America take incredible strides these past seven years," the US president said in a Facebook post released as he took off from Washington.
Obama is due to hold meetings with other world leaders during his two days in Paris, including with Chinese President Xi Jinping and Indian Prime Minister Narendra Modi.
Work for the COP21 summit began to get underway on Sunday, with French Foreign Minister Laurent Fabius laying out a roadmap for negotiations ahead of the official opening.
Negotiators vowed at a preliminary session to honour the 130 people killed by jihadists on the streets of Paris this month by forging an ambitious deal.
"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 -- 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.
"And America's leadership is helping to drive this progress."