More than 20 countries called at UN climate talks on Saturday for the tripling of world nuclear energy capacity by 2050 from 2020 levels.

A declaration endorsed by nations including the United States, Japan, several European countries, and others said nuclear energy plays a "key role" in efforts to achieve net-zero greenhouse gas emissions.

The United States will pledge $3 billion to the Green Climate Fund, sources familiar with the matter said on Saturday as Vice President Kamala Harris arrived in Dubai for the U.N. climate summit.

The fund, with more than $20 billion in pledges, is the largest international fund dedicated to supporting climate action in developing countries.

The latest pledge, which Reuters was first to report, would be additional to another $2 billion previously delivered by the United States.

The sources said the pledge was subject to the availability of funds. Harris was expected to announce the pledge later on Saturday in an address to the COP28 summit.

The fund's facilitators said in October that the current second round of replenishments had brought in about $9.3 billion in pledges to fund projects in climate-vulnerable counties between 2024 and 2027.

Even so, pledges so far represent a fraction of the roughly $250 billion that developing countries would need every year by 2030 just to adapt to a warmer world.

In addition to supporting climate adaptation, the fund also finances projects to help countries shift to clean energy.

A U.S. climate official said the vice president would tell the summit that the world needs to "make sure everybody is at the table, and everybody is stepping up. Everybody has to step up." Harris, who is representing the United States at COP28 in place of President Joe Biden, is part of a U.S. delegation that also includes climate envoy John Kerry and dozens of senior officials.

"It was important for both the president and vice president to ensure that a leader from the United States was at COP," an official said, adding that Harris wanted to "make sure we are telling the world the story of progress that we have made in the U.S."