The Internal Revenue Service announced yesterday that it is cracking down on improper tax deductions taken by people who give real estate and cash to environmental groups, warning that taxpayers could face penalties and charities could lose their tax-exempt status.

The IRS is specifically targeting gifts of "conservation easements" -- deed restrictions that limit some types of real estate development. The easements have become the environmental movement's key tool for preserving fragile ecosystems and millions of acres of open space.

The IRS is focusing on easements that have questionable public benefit or have been manipulated to generate inflated deductions.

"We've uncovered numerous instances where the tax benefits of preserving open spaces and historic buildings have been twisted for inappropriate individual benefit," IRS Commissioner Mark W. Everson said in a statement. "Taxpayers who want to game the system and the charities that assist them will be called to account."

The IRS warned that it intends to levy penalties on charity executives and board members who collect or knowingly help secure improper deductions claimed in connection with such transactions.

The announcement did not name individual taxpayers or charities. It comes as the IRS is conducting a major audit of the Arlington-based Nature Conservancy, the world's largest environmental organization.

The Washington Post reported last year that the Conservancy had repeatedly bought land, added some development restrictions, and then resold the properties at reduced prices to its trustees and other supporters. The buyers made cash gifts to the Conservancy roughly equal to the difference in price, thereby qualifying for substantial tax deductions -- just as if they had given money to their local charity.

The Conservancy said the sales prices were proper because the development restrictions reduced the market value of the tracts. In the wake of the news articles, however, the Conservancy announced that it would no longer conduct such deals with its board members and trustees.

Sheldon Cohen, a former IRS commissioner now working as a private lawyer in Washington, called yesterday's announcement an unusually strong action. He said, "It is pretty obvious who it is aimed at."

Conservancy spokesman James Petterson said yesterday that executives there were studying the IRS action.

"The Nature Conservancy over the last decade has received several legal opinions reflecting other interpretations of the law," Petterson said. "We are reviewing what the IRS issued, assessing its impact on our programs and determining appropriate actions."

In a statement yesterday, the IRS said that it "intends to disallow" and may assess penalties for improper tax deductions claimed for gifts of easements to charities. Easements that serve no conservation purpose and create no significant public benefit do not qualify for tax deductions, the agency said. Some taxpayers have claimed deductions for amounts that exceed the value of the restrictions placed on their land, the IRS added.

The agency also said that in "appropriate cases" it may treat cash payments made to charities coincident with land deals as part of the purchase price -- not as tax-deductible charitable gifts.

"The IRS may impose penalties on promoters, appraisers and other persons involved in these transactions," the release said. "The IRS may challenge the tax-exempt status of the charitable organization, based on the organization's operation for . . . private benefit."

The IRS said one of the agency's top priorities now is fighting abusive tax-deduction schemes involving nonprofit organizations.

The Senate Finance Committee began investigating easement transactions involving the Conservancy and other charities last year. Committee Chairman Charles E. Grassley (R-Iowa) said the investigation's findings so far demand "a serious rethinking" of tax laws and stronger enforcement by the IRS.

"The IRS is right to subject these sweetheart deals, often to insiders, to hard scrutiny," Grassley said yesterday. "I'm encouraged that the IRS is willing to challenge the tax-exempt status of charitable organizations that engage in shady practices in land-donation transactions. Shutting down the bad actors will be a strong signal that 'business as usual' has been put out of business.

"Land donated for a conservation purpose should help the environment or create open space," he said. "All too often, these conservation donations appear to do very little for the environment and only help fill the bank accounts of donors and middlemen."

Rand Wentworth, president of the Land Trust Alliance, called the IRS action "really good news for legitimate charities." The group represents 1,260 nonprofit land banks, many of which hold conservation easements.

"This will help restore the integrity of good land trusts," Wentworth said.

Stephen J. Small, a former IRS lawyer and a leading expert on easements, said he is pleased the agency is targeting appraisers and promoters of improper tax deals. "In this field, this is new," he said. "I think that's great."

Land trusts hold more than 12,000 conservation easements nationwide, though not all of them generate tax deductions for the owners. The IRS said it has no figures for the total value of tax deductions generated by easements.

"Land donated for a conservation purpose should help the environment," Sen. Charles Grassley of the Finance Committee said.