WASHINGTON--Weaker growth in the U.S., China and several important emerging markets forced the International Monetary Fund to downgrade its forecast for the global economy Thursday.

The fund said the world economy should expand by 3.4% this year, down from its April forecast of 3.7%.

The downgrade shows Western nations are still struggling to return to health a half-decade after the financial crisis pushed the global economy into a nose dive. It also underscores how emerging markets are finding it difficult to juice their economies amid a broader slowdown in growth.

"Global growth could be weaker for longer, given the lack of robust momentum in advanced economies despite very low interest rates and the easing of other brakes to the recovery," the IMF said in a quarterly update of its World Economic Outlook.

Governments in both advanced and emerging-market economies must take action to make the economies more competitive, the fund said. "Structural reforms are urgently needed to close infrastructure gaps, strengthen productivity and lift potential growth," it added.

The fund upgraded prospects for Japan, Germany and the U.K., but negative surprises dominated the outlook.

One of the biggest drags was a 1.1-percentage-point cut in the forecast for the world's largest economy, the U.S., from the IMF's April predictions. A bad winter and an inventory overhang drove a 3% contraction in the first quarter.

In China, domestic demand fell more than expected as officials try to rein in credit growth and prevent a real-estate bubble. The fund says it now expects the world's second-largest economy to grow at 7.4% this year, 0.2 percentage point lower than its previous forecast.

The escalating sanctions against Russia for its actions in Ukraine sparked capital flight out of the country, exacerbating a fall in investment levels. The IMF says the Russian economy should grow by only 0.2 % this year, down from its previous forecast of 1.3%.

The fund also cut growth expectations for Brazil and Mexico by more than half a percentage point for the year as weaker U.S. growth and timid investment weigh on growth.

Despite some upward revisions in Europe, high debt levels in Italy and France continue to weigh, particularly as governments have been unable to roll out enough economic changes to convince investors about their longer-term growth prospects. The IMF cut both France's and Italy's outlook by 0.3 percentage point to 0.7% and 0.3% for the year, respectively.

The fund also warned that geopolitical risks over the Ukraine crisis and in Iraq could pressure global growth even further, especially if they lead to a rise in oil prices and trade flows are disrupted. Emerging markets are also at risk if markets shift their expectations for interest rate increases by the U.S. Federal Reserve next year.

Write to Ian Talley at ian.talley@wsj.com

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