A top U.S. commander in Iraq said Tuesday that simmering ethnic tensions between Kurds and Arabs remain a grave threat to Iraq's stability, especially after American forces depart.

The general in charge of U.S. operations in northern Iraq also said that spectacular attacks in Mosul this week show the resilience of Al Qaeda in the city the U.S. has called the last bastion of the terror network.

The competition for energy and land resources between Arabs and Kurds "could certainly resolve in an ethnic, lethal-force engagement," said Army Maj. Gen. Robert Caslen. He spoke to Defense Department reporters through a video hookup from his base in Tikrit.

Several top defense officials have identified the split between Iraq's majority Arabs and the Kurdish minority as probably a greater long-term threat to Iraq's stability than the more familiar Sunni-Shiite conflict. Defense Secretary Robert Gates went to the Kurdish self-rule area in the North to make the case that both sides have limited time to resolve their differences before U.S. troops leave in 2011.

Caslen said he is encouraged by political movement on both sides, particularly Iraqi Prime Minister Nouri al-Maliki's recent trip to see Kurdish leaders on their home ground.

"Are they capable of resolving their ethnic differences peacefully?" Caslen said. "They have the capability to do it. The question is whether the senior leadership will exert the leadership necessary to do that."

The two sides are at odds over borders for the self-rule area and the jurisdiction of the ethnically divided city of Kirkuk.

Addressing the spasm of violence in Mosul and Baghdad, Caslen said al-Qaida is using the new attacks to draw attention, but that so far the Sunni insurgents have not been able to provoke a large-scale sectarian retaliation.

Caslen said the number of insurgent attacks has dropped in Mosul since the handover of control of the city to Iraqi forces on June 30. Weekly attacks averaged 42; they now average 29, Caslen said. At the same time, high-profile attacks are up, attacks on Iraqi security forces are up and the overall number of casualties higher, he said.

A double truck bombing Monday in Mosul and blasts in Baghdad brought the Iraqi death toll to more than 100 in four days, the worst spasm of violence the country has suffered since U.S. forces left the cities.

The bloodshed threatened to chip away at public confidence in the U.S.-backed government as it seeks to project a sense of normalcy ahead of next year's national elections, including an announcement last week that all concrete blast walls will be gone from Baghdad's main roads by mid-September.

Al Qaeda "remain a resilient force that has the capability to regenerate their combat power as necessary," Caslen said.

The Mosul bombing, like another Friday on the fringes of the ethnically tense city, targeted ethnic minorities, indicating that insurgents are seeking out relatively undefended targets to maximize casualties as the strapped Iraqi army focuses its efforts on more central areas.

The attacks in Mosul, which the U.S. military has called the last stronghold of al-Qaida, killed 28 people Monday and 44 on Friday.