Top executives at U.S. oil giants Exxon Mobil (XOM.N), opens new tab and Chevron Corp (CVX.N), opens new tab said on Monday that the U.S. needs to clarify rules on energy subsidies to drive the rapid, large-scale investments needed to fight climate change.

President Joe Biden has been trying to encourage energy producers to slash emissions using technologies like carbon capture and green hydrogen that are expensive and have yet to be proven at scale. Biden’s 2022 Inflation Reduction Act included big subsidies for those technologies, along with incentives for more solar and wind power, and electric vehicles.

“Even today, the IRA has not been translated into regulation. And so, the certainty and the incentives for investing in this space are still being developed and formed,” Exxon CEO Darren Woods told a panel at the Milken Global Conference in Los Angeles.

“And so as a business we've got to understand what are the implications for the investments? What are the returns for our shareholders, making sure it's competitive in our portfolio?” he said.

U.S. climate envoy John Podesta criticized Woods during the panel for "not going fast enough" to cut emissions at Exxon.

Chevron CEO Mike Wirth, also at the conference, said the world was not on track to decarbonize by 2050. Wirth cited the challenge of meeting rising energy demand in developing countries, and added U.S. rules for hydrogen subsidies were also a headwind.

“Treasury is writing the rules and the rules are getting very restrictive, and they discourage investment,” Wirth said. “They make it difficult to get comfortable with a multi-billion dollar investment.. so we've had kind of these conflicting signals.”

Woods and Wirth advocated for global, market-based systems – like a price on carbon - to encourage the advancement and deployment of climate friendly energy technologies.

Woods also said on Monday that the company's acquisition of Pioneer will mean more oil production at lower cost.

"We're bringing a unique set of technologies and development capabilities to a very attractive set of acreage. So we will produce more oil at a lower cost, which is good for the economy," Woods said at the conference.