UPDATE 2-Obama: Gulf drilling only if ensure no more spills

* Obama unveils commission to probe oil spill

* Says companies, government to be held accountable

* Says panel not to interfere with criminal or civil probe (Adds quote from executive order)

By Matt Spetalnick

WASHINGTON, May 22 (Reuters) - U.S. President Barack Obama said on Saturday that offshore oil drilling could only go forward if there were assurances that a disaster like the massive Gulf of Mexico oil spill would not happen again.

As Obama officially unveiled a commission to investigate the accident, he issued a stern message that while keeping pressure on firms involved in the still-uncapped spill -- BP (BP.L), Halliburton (HAL.N) and Transocean Ltd (RIG.N) -- he would also hold Washington accountable for mending its ways.

In his executive order announcing former Democratic Senator Bob Graham and and former Environmental Protection Agency chief William Reilly would co-chair the panel, Obama also made his first reference to the possibility of a separate criminal probe into disaster.

"The commission shall ensure that it does not interfere with or disrupt any ongoing or anticipated civil or criminal investigation or law enforcement activities or any effort to recover response costs or damages arising out of the Deepwater Horizon explosion, fire and oil spill," the order stated.

Attorney General Eric Holder said on May 3 that the Justice Department was part of the investigation into the spill, though a U.S. official at the time said it was not a criminal inquiry.

With frustration growing and political risks looming over the spill, Obama gave the new bipartisan panel six months to report its findings and recommendations for the future of offshore drilling.

"The purpose of this commission is to consider both the root causes of the disaster and offer options on what safety and environmental precautions we need to take to prevent a similar disaster from happening again," Obama said in his weekly radio and Internet address.

Obama has steadily sharpened his tone over the past week as the oil leak -- which threatens to eclipse the 1989 Exxon Valdez spill off Alaska as the worst U.S. ecological disaster -- has spread with little sign it will be halted soon.

Analysts say as ecological and economic damage mounts, it could become more of a political liability for Obama ahead of pivotal congressional elections in November when his Democrats already face the threat of big losses because of voter anxiety over nearly double-digit unemployment.

Creating a commission helps Obama show leadership in a crisis that has drawn heavy criticism not only of companies' safety practices but also loose government oversight.

The spill has raised major questions about Obama's earlier proposal to expand offshore drilling as part of strategy to win Republican support for climate change legislation.

DEMANDS ASSURANCES

Obama said since the spill he had ordered inspections of all deepwater operations in the Gulf and barred permits for drilling new wells until a 30-day review is complete.

"We must ... pursue domestic sources of oil and gas," Obama said. "Because it represents 30 percent of our oil production, the Gulf of Mexico can play an important part in securing our energy future. But we can only pursue offshore oil drilling if we have assurances that a disaster like the BP oil spill will not happen again."

He expressed hope the new commission would "help provide those assurances so we can continue to seek a secure energy future." The seven-member panel is patterned after those that probed incidents such as the 1986 explosion of the space shuttle Challenger.

Obama had more tough words for the companies involved in the spill. "First and foremost, what led to this disaster was a breakdown of responsibility on the part of BP and perhaps others, including Transocean and Halliburton," he said.

"And we will continue to hold the relevant companies accountable not only for being forthcoming and transparent about the facts surrounding the leak, but for shutting it down, repairing the damage it does, and repaying Americans who've suffered a financial loss," he said.

But he also put the onus on government. "Even as we continue to hold BP accountable, we also need to hold Washington accountable," he said. "The question is what lessons we can learn from this disaster to make sure it never happens again."

"If the laws on our books are inadequate to prevent such an oil spill, or if we didn't enforce those laws, I want to know it," he said. "I want to know ... where oversight of the oil and gas industry broke down. We know, for example, that a cozy relationship between oil and gas companies and agencies that regulate them has long been a source of concern." (Editing by Eric Walsh)