BAGHDAD — Iraq opened international bidding for eight enormous oil and gas fields, paving the way for major investments in a nation with one of the world’s largest petroleum reserves.

If the contracts are approved, they could mark the biggest foreign stake in Iraq since the industry was nationalized more than 30 years ago.

That could be good news with the price for a barrel of oil breaching $143 for the first time ever today but there are concerns that a dominant role for Western firms such as Shell, BP, Exxon Mobil, and Total could feed perceptions that American forces toppled Saddam Hussein to grab the country’s natural resources.

During a press conference today, Iraq’s oil minister, Hussain al-Shahristani, named 35 companies that would qualify to bid on service contracts for eight oil and gas fields. The firms included seven from America, three from Britain, and others from countries like Russia and China.

Mr. al-Shahristani said the companies would be invited to bid on the oil fields of Rumeila, Zubair, Qurna West, Maysan, Kirkuk, and Bay Hassan, and the natural gas fields of Akkaz and Mansouriyah.

“These fields were chosen because their production can be raised in a short time and at a low cost,” said Mr. al-Shahristani.

All of the oil fields are currently producing crude, and Mr. al-Shahristani said the new contracts would raise Iraq’s production by 1.5 million barrels per day. Iraq currently produces 2.5 million barrels per day and hopes to raise that to 4.5 million by 2013.

The introduction of 1.5 million barrels of oil each day would likely be enough to move the price for a barrel downward, but analysts are not convinced, given the deterioration of the Iraq’s infrastructure and potential instability, that it is realistic.

“I’m pretty skeptical of that figure,” the president of energy consultancy Ritterbusch and Associates, Jim Ritterbusch, said. “Amount is one thing, timing is another. They still need to upgrade their infrastructure and while things have stabilized, I think you’re assuming a best-case scenario on security and other issues.”

Iraq has been able to boost production to its highest level since the U.S.-led invasion in 2003 because of a reduction in violence.

Iraq’s oil minister said the country needs help from foreign firms to boost production further because some of its oil fields “suffer from old age and need modern technology to check their deterioration.”

Iraq nationalized its oil industry in the 1960s, and participation by foreign firms since then in the development of the country’s natural resources has been limited.

With fuel prices at record levels, foreign firms have been anxious to tap Iraq’s estimated 115 billion barrels of oil reserves and 112 trillion cubic feet of natural gas. The revenue would help the Iraqi government rebuild infrastructure and deliver services to its people.

The deadline for the oil and gas bids announced today is the end of March, and preliminary contracts will be signed next June. Every company involved in the bidding process must have an Iraqi partner and must give 25 percent of the value of the contract to Iraqi companies, said Mr. al-Shahristani.

Debate over Western participation in Iraq’s oil industry reignited about two weeks ago when The New York Times reported that Royal Dutch Shell PLC, BP PLC, Exxon Mobil Corp., Chevron, and Total were close to signing short-term oil service contracts with Iraq on a no-bid basis.

The deals were reportedly designed as an interim way to boost Iraq’s oil output until the country could agree on a new a law on how to divide the country’s oil resources, which has been stalled by political squabbles between the central government and the Kurds.

The Kurdish regional administration in northern Iraq has signed more than 20 oil deals with foreign firms to work in Kurdish-controlled fields since it drafted its own oil and gas law in August 2007.

The Shiite-led Iraqi central government says the deals are invalid with no national oil law in place.

Several Democratic senators in America recently asked the Bush administration to block the Iraqi government’s reported no-bid deals with Western firms until the country finalized the oil law, but the White House refused.

An Iraqi government spokesman, Ali al-Dabbagh, said today the country had never considered a no-bid process, saying “there was never any intention to award the contracts without a tender.”

He also denied American influence on the Iraqi government’s oil decisions, saying “politics does not come into this.”

“There is no preferential treatment for anyone, no matter who,” said Mr. al-Dabbagh.

Major oil companies also distanced themselves from talk of no-bid deals.

“We have been providing services to Iraq from outside the country for a number of years,” a spokesman for BP, Robert Wine, said. “We submitted a study of the Rumeila fields several years ago and if the discussions do lead to deal, they will focus on the technical services in that report. We need to clarify — this is not about access to the country’s oil resources, or exploration. It’s a management contract, to provide technical resources.”

Today the Times reported that a small U.S. State Department team helped draw up contracts between the Iraqi Oil Ministry and the five major oil companies reportedly getting no-bid contracts.

A state Department spokesman, Tom Casey, confirmed a small number of American advisers were providing “technical support” to the Iraqi Oil Ministry. But he said “they are not there to try and give the Iraqis any kind of specific requests or to make decisions or to even push in an individual direction.”

Mr. Casey said the apparent decision by the Iraqis not to announce no-bid contracts for several Western firms today was their own and not influenced by Washington.