Leaders from the world's top economies have broadly agreed at a summit in China to coordinate macroeconomic policies and oppose protectionism, but few concrete proposals emerged to meet growing challenges to globalisation and free trade.

The two-day gathering in the scenic Chinese city of Hangzhou agreed to oppose protectionism, with Chinese President Xi Jinping urging major economies to drive growth through innovation, not just fiscal and monetary measures.

"We aim to revive growth engines of international trade and investment," Mr Xi said in a closing statement.

"We will support multilateral trade mechanisms and oppose protectionism to reverse declines in global trade."

Discussions at the meeting were distracted by North Korea test-firing three medium-range ballistic missiles in a defiant reminder of the risks to global security.

North Korea has tested missiles at sensitive times in the past to draw attention to its military might.

The G20 also called for the formation of a global forum to take steps to address steel excess capacity and encourage adjustments, the White House said in a statement, one of the controversial issues discussed at the summit.

China produces half the world's annual output of 1.6 billion tonnes of steel and has struggled to decrease its estimated 300m tonne overcapacity, and rising prices have given companies there an incentive to boost production for export.