NEW YORK—The terrorism reinsurance market remains vulnerable to a major terrorism loss as the nature of terrorism evolves, according to a report released Tuesday by New York-based Guy Carpenter & Co. L.L.C.

In its report—“Terrorism: Terror Market Continues to Provide Abundant Cover,” Marsh & McLennan Cos. Inc.’s Guy Carpenter unit estimated that approximately $6 billion to $8 billion of terrorism capacity is now available in the U.S. market. In addition, the report found that there is an oversupply of terrorism coverage. That oversupply, combined with the fact that the U.S. government’s terrorism insurance backstop program does not buy reinsurance, will serve to continue to stifle upward pressure on pricing, barring other, significant market-changing events,” said the report.

But the report stresses that the situation could change.

“The reinsurance industry remains vulnerable to a major terrorism loss, since a major market-moving event could quickly shift the current supply/demand dynamic,” a press release about the report said. “Reinsurance capital allocated in support of terror risk is likely to continue to be limited and defined given the uncertain nature of this peril.”

“Although the nature of the threat is very different today from what it was 10 years ago, terrorism remains a constant and serious threat,” said David Flandro, global head of business intelligence at Guy Carpenter, in the statement.

Guy Carpenter had found in June 2010 that reinsurers would like to place more terrorism business, but the demand for standalone terrorism coverage was on the wane.

Developments during the past 12 months, including the killing of al-Qaida leader Osama bin Laden and the political unrest in the Middle East and North Africa, “raise questions” about the future of the international terrorist threat, according to the report.

But new al-Qaida-affiliated groups have emerged in areas such as Yemen. And the recent attack in Norway demonstrates that “the risk posed by domestic militant groups means terrorism is likely to remain a global security threat for the foreseeable future,” the report said.

The number of terrorist attacks peaked at more than 14,400 in 2006, with a “slight dip” in frequency during the past five years, the report said.