WASHINGTON—The U.S. and China on Friday announced significant steps in their efforts to combat climate change, including a pledge by China to launch a program by 2017 to cap some emissions and put a price on carbon and to contribute $3.1 billion to help poorer countries finance their own transition programs.

The announcements showed the two countries’ latest strategies for reaching emissions targets and their efforts to boost an international accord to reduce global carbon-dioxide emissions, which is meant to be wrapped up in December. The Obama administration officials, touting a new era of U.S.-China climate diplomacy, said the statement would demonstrate to the world the countries’ commitment to reaching agreed-upon targets.

China’s plan to finance efforts to combat climate change in poorer countries marks a significant step for Beijing, since previously highly developed economies have borne the burden of funding lower emissions and preparations for adverse weather in poorer countries. China’s contribution—through a specific fund—follows the Obama administration’s pledge last year of $3 billion for the international Green Climate Fund over several years.

“The significant and new climate finance pledges made by China are a game changer in international climate politics,” said Li Shuo, Greenpeace East Asia senior climate policy analyst. “It is a drastic increase from China’s previous finance commitments.”

Mr. Obama on Friday highlighted the U.S.’s plans to reduce carbon emissions while commending China’s decision to begin a national cap-and-trade system and its “major commitment” to climate finance for poorer countries.