Chinese President Xi Jinping told the UN General Assembly Tuesday that his country "will not build new coal-fired power projects abroad" and plans to boost support for clean energy in developing nations.

Why it matters: The pledge, if maintained, would mark a breakthrough in efforts to transition global power away from the most carbon-emitting fuel.

Nations, including the U.S., have been urging China — historically a key source of coal-plant finance — to make such a commitment.

Xi's pledge on coal financing comes just weeks before a critical UN climate summit....