LOS CABOS, Mexico, Feb 20 (Reuters) - Mexico and the United States signed an agreement on Monday to help U.S. firms and Mexican oil monopoly Pemex exploit deep water oil resources in the Gulf of Mexico that straddle the countries' maritime boundaries. The deal, negotiated last year, will lift the moratorium on oil exploration and production in the western gap portion of the Gulf and sets up legal guidelines for companies to jointly develop any trans-boundary reservoirs. "These reservoirs could hold considerable reserves ... but they don't necessarily stop neatly at our maritime boundary. This could lead to disputes," U.S. Secretary of State Hillary Clinton said at a ministerial meeting of Group of 20 nations in Los Cabos Mexico. "The agreement we are signing today will help prevent such disputes." Mexico - the world's No. 7 oil producer - is far behind the United States in exploring for deep water oil reserves and has drilled less than 20 wells in its territorial waters in the Gulf. Pemex estimates there are more than 29 billion barrels of crude equivalent in the area, or 58 percent of the country's prospective resources. Carlos Morales, Pemex's director of exploration and production, told Reuters in an interview on Monday that the company plans to drill six or seven new deep water wells this year and has a plan for eight annually over the next five years. So far most of the discoveries in Mexico's deep waters have been natural gas. Mexico's oil industry regulator worries Pemex does not have the capability or the safeguards in place to move into ultra-deep drilling. Safety concerns loom large after the explosion on BP's Deepwater Horizon rig in April 2010 that killed 11 workers and spewed more than 4 million barrels of oil into the Gulf. Mexico is hoping to attract help from private companies to explore for oil in the region with a 2008 oil reform that allows for incentive based operating contracts, but no projects have yet been announced "This agreement also creates new opportunities for our businesses. American energy companies will be able - for the first time - to collaborate with Pemex, their Mexican counterpart," said Clinton. The country, which relies on oil revenues to fund a third of the federal budget, closely guards its oil resources nationalized since 1938. "This agreement was negotiated invariably under the principle of respecting sovereign rights. Mexican oil wealth is and will continue to be of the Mexicans," President Felipe Calderon said at the event.