The exact details of when U.S. intelligence agencies estimate Iran will have a nuclear bomb are classified, but the timeline for developing a nuclear device has "thankfully slid to the right a bit," he said.

On Tehran's ties to al Qaeda, Gen. Petraeus said the group "continues to use Iran as a key facilitation hub, where facilitators connect al Qaedas senior leadership to regional affiliates."

"And although Iranian authorities do periodically disrupt this network by detaining select al Qaeda facilitators and operational planners, Tehrans policy in this regard is often unpredictable," he stated in written testimony to the Senate Foreign Relations Committee.

The comments reinforce concerns of both military and intelligence officials about Iran's backing for the group behind the Sept. 11, 2001, attacks. Some analysts in the past doubted or minimized links between Shi'ite Islamists like those in Iran and rival Sunni extremists, such as al Qaeda.

A U.S. counterterrorism official said al Qaeda members are located in Iran, and "and the Iranian government knows it."

"Al Qaeda members also make their way through Iran to Pakistan and Afghanistan, and the Iranian government knows it. Thats just unacceptable," the official said.

A National Counterterrorism Center report said Iran is unwilling to "bring to justice" senior al Qaeda members it holds and "has refused to publicly identify those senior members in its custody."

Gen. Petraeus said Iran is also working to undermine U.S. and international efforts to stabilize Iraq, Afghanistan and other areas of the region. He noted that in Afghanistan, Iran appears to have "hedged" its tacit support for the Kabul government headed by President Hamid Karzai by "providing opportunistic support to the Taliban."

In Iraq, Iranian Islamist "Qods Force" personnel are working both politically to influence Iraqi politics and to provide explosives and other military support to Shi'ite militias.

"The Qods Force also maintains its lethal support to Shia Iraqi militia groups, providing them with weapons, funding and training," Gen. Petraeus said.

On the war in Afghanistan, he said the U.S. and allied military surge of forces is in the early stages and aimed at rolling back gains by Taliban insurgents over the past year.

However, the war is far from over and will "likely get harder before it gets easier," he said. Taliban insurgents and al Qaeda extremists, operating from border regions, are waging an increasingly violent campaign in Afghanistan, he said.

The current 12- to 18-month campaign is seeking to retake the initiative from the insurgents.

Gen. Petraeus said the new strategy unveiled in 2009 of more troops, civilian resources and greater international involvement "can help turn the tide over time, but we must manage expectations as we continue the buildup in our forces," the four-star general said.

"Progress will be incremental and difficult," he said.

He also warned that the growing stability in Iraq remains "fragile" despite significant gains. Fundamental issues such as the distribution of political power and resources are unsettled, and the developing state faces "numerous challenges," Gen. Petraeus said.

Al Qaeda has its main operating bases in the ungoverned regions between Afghanistan and Pakistan, and while "under pressure" it continues to threaten the stability of nuclear-armed Pakistan, he said.

The prospect of "significant instability" in Pakistan poses a serious threat, a result of the country being a "a critical strategic foothold for al Qaeda and … important to the organization's efforts to rally supporters worldwide."