Iraqi security forces are probably incapable of retaking large stretches of territory seized by Sunni insurgents in recent weeks without outside help, the Pentagon’s top leaders said Thursday as they sketched a bleak assessment of turmoil in the country and forecast a protracted conflict that would be difficult to contain.

Defense Secretary Chuck Hagel and Army Gen. Martin E. Dempsey, the chairman of the Joint Chiefs of Staff, said at a news conference that U.S. commanders are still considering what potential military courses of action they will recommend to the White House and whether U.S. troops should take a more active role. But they said deeper U.S. involvement would hinge on Iraq’s ability to overcome deep-seated political and sectarian fissures and form a national unity government.

Dempsey said the Iraqi army had “stiffened” its resistance to a fast-moving insurgency led by the Sunni fighters of the Islamic State of Iraq and Syria, which has renamed itself Islamic State and declared a caliphate on land it controls. He said government forces were “capable of defending Baghdad” but added that they would be challenged to go on the offensive without external support.

Dempsey said the U.S. government was contemplating the possibility of airstrikes, as well as providing help beyond the roughly 750 U.S. military advisers and other troops now in Iraq, most of whom have arrived in the past three weeks. “What will we be willing to contribute to that cause?” he said. “That’s not a question that we’re prepared to answer just yet.”

The answer, he added, would depend on political developments in Iraq. Prime Minister Nouri al-Maliki, a Shiite Muslim, has alienated Sunnis, Kurds and other groups, and it is unclear whether he can muster the votes in parliament to stay in office.

If Iraq is unable to form a new government that can draw support from all factions, “then the future’s pretty bleak,” Dempsey said, suggesting that it would be pointless for the U.S. military to take sides without a reliable partner.

The United States is not the only foreign power with a direct military stake in Iraq. Neighboring Iran, whose Shiite leaders are closely allied with Maliki, has deployed additional forces and advisers and even aircraft to Iraq. In recent days, Russia has sent used fighter jets and a small number of military advisers.

In their first joint news conference at the Pentagon since February, Hagel and Dempsey sidestepped questions about whether the Obama administration might send more troops to Iraq, what they might do and how long they might stay there.

The defense secretary opened his remarks by saying, “President Obama has been very clear that American combat troops are not going to be fighting in Iraq again.” But when asked how that could be the case if Obama orders airstrikes in Iraq — which probably would be guided by U.S. forces on the ground — Hagel said he was referring only to the U.S. advisers and assessment teams now in the country and acknowledged that their mission could change.

“We have one mission today, and that’s assessments,” Hagel said. “I don’t know what the assessments are going to come back and say or what they would recommend.”

On June 18, during testimony before Congress, Hagel and Dempsey played down the possibility of a U.S. bombing campaign in Iraq to weaken the terrorist organization. They questioned the strategic objectives of airstrikes and said Sunni insurgents had blended into the local population, raising the likelihood of civilian casualties.

On Thursday, Dempsey said that intensified surveillance flights and the deployment of U.S. advisers and liaison officers had improved the Pentagon’s view of the battlefield. But he added that it was still difficult to sort out hard-core Islamic State fighters from disaffected Sunnis opposed to Maliki’s rule.

“We have a much better intelligence picture than we did two weeks ago, and it continues to get better,” Dempsey said. “The complexity, though, is the intermingling of [Sunni] groups. . . . And that’s going to be a tough challenge to separate them, if we were to take a decision to strike.”

The Army general, who served multiple tours as a commander in Iraq, drew a broad outline of what it would take to reverse the insurgents’ gains. He said Islamic State was “stretched” after taking control of so much territory and indicated that the group’s supply lines were vulnerable to disruption.

“In any military campaign, you would want to develop multiple actions to squeeze” Islamic State, Dempsey said. “You’d like to squeeze them from the south and west. You’d like to squeeze them from the north and you’d like to squeeze them from Baghdad.”