NEW YORK - The United States is still working toward an agreement with G20 partners to phase out subsidies for fossil fuels, a top White House adviser said ahead of this week's G20 summit.

Michael Froman, deputy national security adviser and top G20 aide to President Barack Obama, said the United States was hoping to reach an agreement about the issue at the Pittsburgh summit on Thursday and Friday.

We've put on the table the desirability of reaching an agreement to phase out fossil fuel subsidies, Froman told reporters in remarks embargoed for release on Wednesday.

We're working with the rest of the G20 to see if we can forge an agreement that would make a significant contribution in that direction.

Froman declined to flesh out the U.S. ideas by including a timeframe or identifying which countries were targeted.

A source familiar with the proposal said earlier this month it would seek to phase out subsidies in five years.

The proposal -- which could rankle G20 states with big fuel subsidies like China, Russia, and India -- argues non-G20 members should end subsidies by 2020, the source said.

Froman laid out the U.S. case in broad terms, saying lower consumption of fossil fuels that results from eliminating subsidies would help combat climate change, heighten energy security, improve health and the environment, boost economic growth, and assist the poor.

Citing estimates by the Organization for Economic Cooperation and Development and the International Energy Agency, Froman said phasing out fossil fuel subsidies worldwide would cut greenhouse gases by up to 12 percent by 2050.

In his speech to a U.N. climate summit on Tuesday, Obama said he planned to work with his G20 counterparts this week to phase out the subsidies.

Indonesia, which was considered a success story in phasing out fossil fuel subsidies, had agreed to open the discussion among G20 leaders, a U.S. official said.

Froman said the United States encouraged subsidies and support schemes for renewable energy.

The United States is also seeking G20 support for boosting transparency in oil markets.

G-20 Leaders should commit to improving energy security by increasing oil market transparency, including by reporting comprehensive data on domestic oil markets, Froman wrote in a letter, obtained by Reuters, to G20 colleagues.