U.S. lawmakers on Thursday sought to increase incentives for nuclear power and energy efficiency in a measure that would require utilities to generate a certain amount of electricity from renewable sources.

Nuclear power is not currently considered a renewable electricity source in the Senate Energy and Natural Resources Committee bill. Under the bill, a percentage of utilities' total power production would have to be dedicated to renewables.

The committee adopted an amendment offered by Ranking Member Lisa Murkowski that excluded any increases in capacity at existing nuclear power plants and new nuclear plants from measures of utilities' total production for the renewable electricity standard.

By not counting nuclear upgrades within utilities' overall electricity output, the amendment effectively allows power plants to increase energy production without also increasing the amount of renewable power those plants must generate.

Other nuclear power amendments were voted down by the panel, including one offered by Republican Senator John McCain of Arizona. McCain's measure would have counted all nuclear power as renewable energy.

Reduced greenhouse gas emissions, have cleaner sources of energy and diversity: I certainly think nuclear power meets all of those definitions, McCain said.

The proposed bill would require power plants meet targets to gradually produce more renewable power, beginning with 3 percent of their output between 2011 and 2013 and rising to 15 percent between 2021 and 2039.

Utilities could meet about a quarter of their renewable requirements through energy efficiency gains. The panel voted down an amendment offered by Democratic Senator Mary Landrieu of Louisiana that would have removed the cap on how much energy efficiency can be used to comply with the renewable standard.

We wanted to incentivize renewable power because it was seen as best way to clean our air and reduce our greenhouse emissions. It's become clear, however, we can do even better and more cheaply by improving efficiency in our existing generation, Landrieu said.

Other lawmakers said the renewable energy targets in the proposed bill were not strong enough. Committee chairman Jeff Bingaman's initial draft would have required 20 percent of power from renewable energy in 2021-2039.

It seems to me we've come a long way in the wrong direction since November, said Democratic Senator Robert Menendez of New Jersey.

Menendez is co-sponsoring an amendment authored by Democratic Senator Mark Udall of Colorado that would direct utilities to produce 25 percent of electricity from renewable sources by 2025. The Senators plan to offer their measure before the full Senate.

The renewable power mandate would be folded into a larger energy package that would address various issues including building efficiency, expanding electricity transmission and domestic energy production. The committee plans to vote on the complete package next week.

(Editing by Christian Wiessner)