To ensure development is sustainable, the quest for global economic growth must coexist with environmental protection and social progress, the head of the International Monetary Fund warned on Tuesday, ahead of an upcoming UN conference.

"We are facing a triple crisis - an economic crisis, an environmental crisis and, increasingly, a social crisis," IMF Managing Director Christine Lagarde said in a speech at the Center for Global Development in Washington.

"The global economy is still rocked by turmoil, with uncertain prospects for growth and jobs," she told the audience. "The planet is warming rapidly, with unknown and possibly dire consequences down the line. Across too many societies, the gap between the haves and have-nots is getting wider and strains are getting fiercer."

As these threats feed off each other, she urged governments to pursue joint solutions by restoring financial stability, accurately pricing energy including renewable sources and promoting inclusive growth.

The United Nations Conference on Sustainable Development, dubbed Rio+20, is set for Brazil on June 20-22. It will mark the 20th anniversary of the 1992 UN "Earth Summit" in Rio de Janeiro, and the 10th anniversary of the 2002 World Summit on Sustainable Development in South Africa.

The 1992 meeting produced the action plan Agenda 21, the Climate Change Convention and the Convention on Biological Diversity.

It will be the first time since then that the world body has organized an event on this scale, combining the environment, development and economic policy.

World leaders, along with thousands of participants from governments, private sectors and civil societies, will gather in Rio to discuss ways to reduce poverty, advance social equity and ensure environmental protection, according to the event's website.

Sustainable development must spring from macroeconomic and financial stability, which in turn pave the way for robust growth and a productive economy, Lagarde said.

She asked European policymakers to take decisive steps to break free of current financial crises.

"First, they need to rekindle demand today, to get the growth engine up and running again. Second, they must make sure that any spark to demand today leads to sustained growth tomorrow, which means reforms on the supply side to boost the productive capacity of the economy."

The IMF will continue to raise resources for concessional lending, to help vulnerable countries navigate an increasingly volatile world, she added. And it will help governments use fiscal policy to address environmental challenges.

"Perhaps we can help with a simple concept that everybody can understand: getting the prices right," Lagarde said. "Getting the prices right means using fiscal policy to make sure that the harm we do is reflected in the prices we pay."

This would include governments taxing emissions of carbon dioxide to help limit human-generated greenhouse gases that contribute to changes in the Earth's climate, and establishing systems for buying and selling "pollution rights", she said.