With the leaders of the United States and China listening to him, South Korean President Moon Jae-in called Friday for a concerted and responsible role by the Group of 20 member states in ending global trade wars and other economic challenges.

"The G-20 should show leadership again," Moon stressed, speaking at the first session of this week's annual summit here on world economy, trade and investment.

He pointed out that the world is confronted with new challenges amid growing uncertainties and downside risks facing the global economy.

The IMF and the OECD have lowered the growth forecasts for the world's economy, citing trade disputes and the spread of protectionism as a main factor, he said.

It's even hard to predict the future on the threshold of the "New Abnormal" era beyond the "New Normal" one, he said, according to a transcript of his speech released by Cheong Wa Dae.

"Such challenges can't be resolved on an individual country level," Moon said.

He called on G-20 members to cope with the downside risks of the world economy in a preemptive manner, saying his administration has adopted an expansionary fiscal policy.

What's also very important is to solidify the global financial safety net to handle the uncertainty of the market, he was quoted as saying.

Moon reaffirmed that South Korea with Asia's fourth-largest economy will participate actively in efforts to reform the World Trade Organization in a bid to promote fair trade.

The G-20 summit opened at the INTEX convention center in Osaka earlier in the day, attended by the leaders of 19 nations and the European Union, who include US President Donald Trump and Chinese President Xi Jinping.

The group represents more than 80 percent of the world's economic output.

Moon said that under his administration, launched in May 2017, South Korea has placed an emphasis on two pillars: innovation and inclusiveness.

On the sidelines of the multilateral forum, meanwhile, Moon had bilateral summits with Indian Prime Minister Narendra Modi, Indonesian President Joko Widodo and French President Emmanuel Macron on Friday.

Moon will also hold one-on-one talks with Russian President Vladimir Putin at a downtown hotel later in the day.

Moon was briefly greeted by Japanese Prime Minister Shinzo Abe at the official welcoming ceremony. But it remains unconfirmed whether they will have separate talks with each other before Moon returns to Seoul on Saturday afternoon.

Moon's aides said earlier that he has proposed talks with Abe but that Japan has not responded.

Seoul-Tokyo relations have chilled amid disputes over compensation issues associated with Japan's brutal colonization of Korea from 1910-45. (Yonhap)