Al Qaeda is still plotting and preparing attacks on the United States, though great strides have been made to degrade the terror organization, National Intelligence Director John Negroponte told a Senate panel on Thursday.

Speaking on behalf of the entire U.S. intelligence community in a rare open session, Negroponte told the Senate Select Committee on Intelligence that Al Qaeda and Iran are this nation's top security concerns.

"Although an attack using conventional explosives continues to be the most probable scenario, Al Qaeda remains interested in acquiring chemical, biological, radiological and nuclear materials or weapons to attack the United States, U.S. troops and U.S. interests worldwide," he added.

Negroponte also said 40 terror groups, insurgencies or cults have obtained or are trying to get a hold of chemical, biological or nuclear weapons. He added that Tehran already has the "largest inventory of ballistic missiles in the Middle East," and views its acquisition of nuclear weapons capabilities an inevitable and necessary pursuit.

Despite his conclusion that Tehran is not equipped with a nuclear weapon yet, Negroponte said the danger that it will acquire one is reason for "immediate concern." The director of national intelligence (DNI) also said North Korea probably does have nuclear weapons.

"Despite its claim to the contrary, we assess that Iran seeks nuclear weapons. We judge that Tehran probably does not yet have a nuclear weapon and probably has not yet produced or acquired the necessary fissile material," he said.

Negroponte spoke as U.S. and European diplomats work toward building a consensus on referring Iran to the U.N. Security Council over concerns that it is seeking nuclear weapons. The International Atomic Energy Agency's 35-nation board of governors began a two-day meeting Thursday on a European draft resolution calling for Tehran to be referred to the Security Council, which can impose sanctions.

Negroponte said the United States has made significant progress in eliminating those who were in control of Al Qaeda back in 2001, has depleted its cadre of followers and has continued to disrupt terror operations. But he warned that the terror group today, while "battered," is still "resourceful."

He suggested that "high impact attacks" would continue, and the United States is "more likely to see attacks from terrorists ... than from states, although terrorist's capabilities would be much more limited."

Still, he cited recent tapes released by Al Qaeda members Usama bin Laden and Ayman al Zawahiri as evidence that the terror group has the ability to communicate with its followers.

As for Iraq, Negroponte said strong Sunni participation in the December elections is a step toward diminishing support for the insurgency, but the DNI warned that if the terrorists succeed in thwarting security and disrupting a stable political environment from growing, they could secure an operational base there.

"Even if a broad inclusive national government emerges, there almost certainly will be a lag time before we see a dampening effect on the insurgency," Negroponte said, calling Sunni disaffection the "primary enabler" of the insurgency there.

In a separate appearance, Defense Secretary Donald H. Rumsfeld said the United States has made strides in the War on Terror, but that the threat may be greater than ever because the available weapons are far more dangerous.

"The enemy, while weakened and under pressure, is still capable of global reach, and still possesses the determination to kill more Americans — and to do so with the world's most dangerous weapons," he said in remarks prepared for the National Press Club.

Negroponte's testimony was his first before a congressional committee since his confirmation hearing last April. He was made director of national intelligence after Congress created the position upon the recommendation of the Sept. 11 commission. The DNI's job is to coordinate the work of the government's 15 intelligence agencies.

Thursday's hearing represented a major departure for the protocol in which administrations have publicly briefed Congress on intelligence issues.

In years past, the heads of the CIA, FBI and Defense Intelligence Agency have offered their own independent analysis on global and domestic threats to the intelligence committee. Providing a stark reminder that Negroponte is now in charge, he decided to change the format Thursday, and delivered one unified assessment for all U.S. spy agencies.

Former Indiana Democratic Rep. Tim Roemer, a member of the Sept. 11 commission, said he does not "feel comfortable enough that we're making the strides to re-organize" the government and its assets to address constantly-changing terror threats.

"Certainly Al Qaeda is patient ... We have to be ever-vigilant, we have to be almost perfect, we have to think outside the box," in order to control the recruitment of terrorists in the future, he said.

Suggesting that "jihadists are spreading out like mercury on a mirror across the globe today," Roemer said winning the hearts and minds will go a long way to making sure "the jihadists aren't getting on the conveyor belts to come at us in the future."

"I think Al Qaeda is being degraded and eroded, I think that is one of the successes over the last couple years. However, Al Qaeda is turning into Abu-Sayyef in the Philippines or Jemaa Islamiya in Indonesia or radical Islamic groups in the Africa and the Middle East. So we have to be as effective ... at going after them, attacking them, getting new military weapons in the Defense Department that can go after asymmetrical threats," he said.

Negroponte said intelligence reforms have helped make the United States be more prepared for attacks, but it cannot rest — further integration among the intelligence agencies, local law enforcement and U.S. forces overseas is key.

Based on questions by Democratic lawmakers at the hearing, some senators seemed more concerned with accusing the Bush administration of exceeding the bounds of its power — from allegedly manipulating the evidence that led to the war in Iraq to failing to come clean with the Senate Intelligence Committee about the National Security Agency's warrantless wiretap program.

Sen. Ron Wyden, D-Ore., asked Negroponte what happens to information on an individual whose conversations are intercepted but who is later determined not to be a threat. Negroponte said strict procedures are followed with legal and other oversights. Wyden wasn't impressed.

"That answer is essentially, 'trust us. The Congress and the public just have to trust us.' And Ronald Reagan put it very well. He said 'trust but verify.' And we have no way to verify that citizens are being protected the way you have outlined today," Wyden said.

"The question I am wrestling with is whether the very independence of the U.S. intelligence committee has been co-opted — to be quite honest about it — by the strong, controlling hand of the White House," added Sen. Jay Rockefeller, D-W.Va., the ranking Democrat on the panel.

Rockefeller told Negroponte it was unacceptable that the White House refused to share information on its warrantless eavesdropping program, in which phone calls and e-mails of those with suspected Al Qaeda ties are monitored even if one person involved in the communication is inside the United States.

"What is unique about this one particular program among all the sensitive programs that justifies keeping this committee in the dark?" he asked.

The White House has said the top Republican and Democratic leaders in the Senate and House chambers and on their respective intelligence committees have been periodically updated on the program started shortly after Sept. 11, 2001. The NSA program was first exposed by The New York Times late last year.

FOX News' Megyn Kendall and The Associated Press contributed to this report.