Sen. Joe Lieberman on Sunday urged Gen. David Petraeus to change the rules of engagement "as soon as possible" for U.S. troops in Afghanistan, saying the strict policy has "hurt morale" among American military.

The Connecticut independent senator, speaking from Kabul on "Fox News Sunday," said the incoming commander told him he was "committed" to reviewing the rules. Those rules, put in place by outgoing Gen. Stanley McChrystal, are classified but generally aim to limit civilian casualties by prohibiting troops from firing unless they're shot at -- or from launching bomb or artillery attacks when civilians are near the target.

Lieberman acknowledged that civilian casualties damage the counterinsurgency campaign U.S.-led troops are trying to wage, but said the policy has also put American troops in harm's way.

"Ultimately, we've got to be concerned about the safety of our American troops here," Lieberman said. He said he's heard stories about troops having to wait too long to get air support when under fire. "We can't let that happen."

Lieberman is in Afghanistan with Sens. John McCain, R-Ariz., and Lindsey Graham, R-S.C. He said he and his colleagues "encouraged" Petraeus to take a hard look at the engagement policy as he assesses the situation in his new command.

"I hope he changes those rules, clarifies them as quickly as possible," Lieberman said.

A military source told Fox News shortly after McChrystal resigned that Petraeus was planning to modify the engagement policy. Other sources said that while the general would review the rules it was too early to say whether he'd make any changes to them.

Petraeus, who left his position as head of U.S. Central Command, formally assumed command in Afghanistan on Sunday. McChrystal resigned after he and his aides made controversial comments that were featured in a Rolling Stone magazine article last month.