PARIS (AP) – President Barack Obama on Monday touted close U.S. cooperation with China on climate change as vital to world efforts to slow global warming, even as he acknowledged persistent differences with China’s President Xi Jinping over cybersecurity and maritime security.

Huddling with Xi on the sidelines of the Paris climate conference, Obama said nowhere had coordination with Beijing been more critical or fruitful than on climate change. He credited U.S. and Chinese leadership with leading 180 nations to make their own pledges to curb emissions in the run-up to the Paris talks.

“Our leadership on this issue has been absolutely vital,” Obama said.

Yet Obama also invoked hot-button issues that have long vexed U.S.-China relations as he and Xi opened their meeting. The United States has complained bitterly about cyber-attacks against the U.S. emanating from China, and Obama has spoken out regularly against China’s assertive moves in disputed waters in the East China Sea and South China Sea.

Obama said the U.S. and China had developed a “candid way of discussing these issues,” adding that he hoped to build on that tradition during his meeting with Xi. “Our teams have found ways to work through these tensions in a constructive fashion,” he said.

The global response to the attacks that killed 130 here two weeks ago have cast a somber mood over the talks, and Obama encouraged China to play a bigger role addressing the crisis, and specifically called for Beijing’s help in securing a political transition in Syria. He expressed condolences over the killing of a Chinese hostage by IS and said it “indicates that this is a threat to all of our nations.”

The Chinese leader, speaking through a translator, said that global worries made it even more important for the U.S. and China to work together. He called for the U.S. and China to build a new model of cooperation, using diplomatic language long preferred by Beijing.

“The world economy is recovering slowly, terrorism is on the rise, and climate change is a huge challenge. There is more instability and uncertainty in international situations,” Xi said.

Obama’s meeting with Xi at the opening of the two-week Paris conference was designed to underscore the need for all nations to come together on a strong agreement to combat climate change. China emits about 30 percent of the world’s greenhouse gases and the U.S. about 16 percent.

Both countries have sought to use their cooperation on the issue to ramp up pressure on developed and developing countries alike to pledge ambitious cuts. In a joint announcement last year, Obama pledged to cut U.S. emissions up to 28 percent over the next decade, while Xi announced China would cap its growing emissions by 2030 or sooner.

As the conference kicked off, the U.S. State Department announced early Monday that the U.S. was pledging $51 million to a global fund to help poorer countries adapt to climate change. The U.S. contribution joins pledges from Germany, Canada, Italy and others to total $248 million.

The Obama administration didn’t specify where the U.S. dollars would come from. Obama has struggled to convince the Republican-run Congress to fund his climate goals, amid concerns from critics in the U.S. who say his energy plan is unattainable and could be wiped away by his successor.

During the two days of talks, Obama was slated to give a brief speech about the budding agreement and meet with Indian Prime Minister Narendra Modi. He also planned a dinner with French President Francois Hollande and a meeting with island nations hit hard by climate change before returning to Washington on Tuesday.