US regulators proposed Friday that companies using fracking to explore for natural gas must disclose the chemicals they use, but only after they drill -- sparking more ire from environmental activists.

Under pressure to tighten regulation of hydraulic fracturing, the technique behind the boom in US gas production, the Interior Department proposed that drillers must reveal what chemicals they pump into the ground when working on public and Native American lands.

In an update of 30-year-old regulations, drillers will also have to do more to prevent wells from leaking chemicals and gas, and to manage better the use of water in the process.

The new rule would cover about 756 million acres (306 million hectares) of public and Indian land, a relatively small portion of all the land in the country where fracking takes place. State and private lands are not covered in the regulations.

Fracking involves pumping liquids and chemicals deep underground at high pressures to fracture rock strata and release trapped gas deposits.

Critics say the process risks the chemicals polluting ground water resources.

But the oil and gas industry argues the chemicals they use are competitive secrets, and that the risk of ground water contamination is unproven.

"This common-sense measure... supports the continued development of America's abundant oil and gas resources on federal and Indian lands by taking steps to ensure public confidence in well stimulation techniques and technologies, including hydraulic fracturing," the department said in a statement.

The regulations, which could come into effect by the end of the year after a period of public comment, would push up the average annual cost per well by $11,833, the department said.

Jessica Ennis of Earthjustice said the new rules "fall far short of what's needed to protect public health."

"In light of the near-constant reports of fracking-related air and water pollution, an update to federal rules is long overdue."

But, she added, the industry should be forced to disclose the chemicals before they are deployed.

"This information is essential so communities can test drinking water before fracking occurs and monitor the safety of water supplies in real time. If there's a problem with their water, families deserve to know immediately -- not after they've been drinking it for years."

An Interior official said the department found "little practical benefit" to requiring disclosure prior to drilling, and that it could hold up the start of wells.