WASHINGTON - The United States intends to work toward reducing emissions of potent greenhouse gases found in refrigerators and air conditioning systems but has not yet decided which international venue to use to advance the issue, U.S. officials said on Monday.

In a letter to a U.N. agency, the U.S. State Department said hydrofluorocarbons, known as HFCs, pose a very significant further threat to the climate system because of their high global warming potentials.

The letter, by Deputy Assistant Secretary of State Daniel Reifsnyder, indicated that the United States wanted to reduce the gases but had not decided whether to do so under the Montreal Protocol, which regulates hydrochlorofluorocarbons (HCFCs), which are blamed for depleting the ozone layer, or through separate U.N. talks on climate change.

The United States has been extremely interested in how best to address the projected future growth of HFCs and how to promote the development of technically and economically feasible alternatives, the letter said.

While uses now of HFCs are relatively small, they are projected to increase dramatically in future years as (countries) transition out of HCFCs and as the market for air conditioning and refrigeration in developing countries grows.

Parties to the Montreal Protocol on Substances that Deplete the Ozone Layer decided in 2007 to speed up plans to phase out ozone-depleting HCFCs, but the most likely alternative to these gases are climate-warming HFCs.

Thus, we risk solving one global environmental problem while possibly exacerbating another unless alternatives can be found, the letter said.

A senior administration official said the letter was meant to signal a U.S. desire to move forward on the issue despite not having decided on the best venue.

The purpose for doing this is to signal that we're really serious about reducing HFCs, the official, who asked not to be named, told Reuters.

Nations are meeting in Copenhagen in December to hammer out a climate treaty to take over for the Kyoto Protocol, which curbs greenhouse gas emissions. A Copenhagen agreement could also cover HFCs.

Reifsnyder wrote that Washington had not yet decided whether the Montreal treaty should be amended to gradually reduce HFCs but emphasized a U.S. interest in addressing their expected growth promoting alternatives.

We are conscious that alternatives exist today for some uses of HFCs, but not all. For this reason, seeking to phase down the consumption and production of HFCs would be preferable to phasing out such consumption and production, he said.