An Iraqi extremist group whose main base was destroyed by U.S. and Kurdish forces last spring has emerged as the key terrorist threat to American forces in Iraq, a senior U.S. general said Thursday.

Lt. Gen. Norton Schwartz (search), director of operations for the Pentagon's joint staff, said American forces in Iraq are concentrating on Ansar al-Islam (search), a shadowy group that the United States calls a terrorist organization with links to Usama bin Laden's Al Qaeda network.

Schwartz referred to the group as "our principal organized terrorist adversary in Iraq right now."

He noted that one of Ansar's highest-ranking members, Aso Hawleri (search), was captured recently by soldiers of the U.S. Army's 101st Airborne Division near the northern Iraqi city of Mosul. He would not say whether interrogations of Hawleri have provided any useful information.

In a memo to four of his top advisers last week about progress in the global war on terrorism, Defense Secretary Donald H. Rumsfeld (search) wrote: "With respect to Ansar al-Islam, we are just getting started." He did not elaborate, and his chief spokesman, Lawrence Di Rita, declined to discuss it further when asked Thursday why the effort against Ansar was just now beginning.

The Pentagon has no official estimate of the size of Ansar al-Islam, but its members are thought to number in the hundreds rather than thousands. In a paper published in December 2001, Iraq expert Michael Rubin said they had about 400 fighters — including some trained in Afghanistan — and said the group, known then as Jund al-Islam, received a $300,000 grant from bin Laden.

It remains unclear whether Ansar has played a role in any of the recent terror-style bombings in Iraq.

During the period it operated in northern Iraq, before the U.S. invasion, Ansar's tactics were known to have included suicide bombings, car bombs and assassinations of political figures.

Before the war, Ansar had taken control of a slice of the Kurdish-controlled area near the Iranian border, enforcing a version of Islam only slightly less stringent than the Taliban in Afghanistan.

Schwartz said Ansar fighters are now present in Baghdad and elsewhere in Iraq. He cited indications of linkages between senior Ansar leaders and fugitive Baath Party members loyal to Saddam Hussein, but added that "generally speaking they are independent actors."

Some Ansar fighters were known to have fled across the border into Iran when their main base was attacked early in the war. Neither Schwartz nor Di Rita would say whether Iran, which was said to have imprisoned some of the Ansar fighters, released them to return to Iraq.

"There's no question that countries surrounding Iraq have in some case been certainly abetting the passage of terrorists into Iraq, and Ansar may be one of them," Di Rita said.

Schwartz said U.S. troops and Iraqi border police are now making a major effort to secure Iraq's frontiers with Syria, Jordan, Iran, Kuwait and Saudi Arabia to minimize the influx of terrorists.

Describing the broader security situation in Iraq, Schwartz said the number of attacks against American forces by resistance fighters has grown to about 25 per day, an increase of between five and seven from several months ago.

The U.S. military's description of the trend in attacks has varied recently. On Wednesday, the top U.S. commander in Iraq. Lt. Gen. Ricardo Sanchez (search) said the number had jumped recently to 35 per day, while on Thursday another military official put the number at 26.

Rumsfeld, meanwhile, sought to inject a little humor into the storm of controversy triggered by publication on Wednesday of an internal memo in which he said U.S. and coalition forces in Afghanistan and Iraq face a "long, hard slog." Some in Washington interpreted that as revealing a greater sense of doubt about progress on the front lines of the war on terrorism.

In a brief appearance before reporters at the Pentagon, Rumsfeld said he had found a dictionary that defined the term "slog" as meaning "to hit or strike hard; to drive with blows; to assail violently." That, he said, is exactly what U.S. forces are doing and will continue to do.

When a reporter noted that another dictionary defines "slog" as meaning to plod, "as in 'slog across the swamp,'" Rumsfeld allowed that he knew that definition but liked the other better.