The world economy is set to grow 3.0 percent this year and 3.6 percent next year, the Paris-based Organisation for Economic Cooperation and Development said in an update of its forecasts for major economies.

It trimmed its estimates from 3.1 percent and 3.8 percent in June, citing primarily a slowdown in emerging market economies like China and Brazil.

"Global growth prospects have weakened slightly and become less clear in recent months," OECD chief economist Catherine Mann told Reuters in an interview.

The United States stood out as a bright spot. The OECD raised its growth outlook for this year to 2.4 percent from 2.0 in June. It lowered its 2016 forecast to 2.6 from 2.8 percent previously, though.

The OECD saw more arguments in favour of the Federal Reserve raising interest rates than standing pat when its policymakers meet this week.

"Raising interest rates now would remove uncertainty in the markets," Mann said. The pace of future increases was more important than whether the Fed acted , according to the OECD's simulations, she said: "The path matters four times as much as the timing."

Looking at the euro zone, its outlook was the brightest in four years. Its growth was projected at 1.6 percent this year and 1.9 percent next year.

However, the bloc should be growing as much as a full percentage point faster, Mann said, with a weak euro and low interest rates and oil prices in its favour. It was not because it remained too burdened by its debts, she said.

The euro zone's priority should be repairing the banking system and tackling bad loans, she said, rather than extending or expanding the European Central Bank's bond-buying programme as some economists have suggested following recently weak inflation data.

The OECD slightly lowered its growth estimate for China to 6.7 percent for this year and 6.5 percent next year after a string of disappointing data and plunges on its stock market.

Brazil was a particularly weak spot in the global economic outlook. The OECD forecast its economy would contract 2.8 percent this year and 0.7 percent next year as it struggles with a collapse in the price of commodities it exports.