The United Nations is forecasting that the global economy will expand by more than 3 per cent this year and next year -- but it warns that increasing risks could trigger "a shock to investment and trade" and a sharp drop to 1.8 per cent growth in 2019.

The UN's mid-year report on the World Economic Situation and Prospects launched Thursday says growth in the world economy is surpassing expectations, reflecting further growth in developed countries and broadly favourable investment conditions.

However, the report said, "downside risks" have increased including "a rise in the probability of trade conflicts between major economies."

A UN official cited the Trump administration's imposition of tariffs in January and proposed new tariffs against China, but also the renegotiation of the U.S. trade agreement with Mexico and Canada.