Most U.S. and NATO combat troops have resumed joint operations with Afghan forces, the Pentagon said Thursday, although U.S. officials said they remain worried about the threat of fratricidal “insider attacks.”

U.S. commanders had substantially scaled back the joint operations 10 days ago in an urgent effort to reduce the vulnerability of U.S. and NATO troops.

At a news conference, Defense Secretary Leon E. Panetta said “most” U.S. and NATO units had “returned to their normal partnered operations” with their Afghan allies. But he offered few details, and other Pentagon officials offered conflicting accounts of how many missions were still being conducted separately.

A spike in the number of insider attacks has undercut the U.S. strategy to end the war by training hundreds of thousands of Afghan soldiers and police to take over responsibility for fighting the Taliban and other insurgents. The Obama administration has pledged to end the current U.S. combat mission by the end of 2014.

Panetta said that the strategy remained sound but that the U.S. military would “take whatever steps are necessary to protect our forces.” At least 51 NATO troops — most of them Americans — have been killed this year by insider attacks, accounting for about one in every five combat deaths for the foreign forces.

“I expect that there will be more of these high-profile attacks and that the enemy will do whatever they can to try and break our will using this kind of tactic,” Panetta said. “That will not happen.”

Military commanders said they have adopted new precautions to prevent and deter such attacks, including better screening of Afghan recruits. But they acknowledged an intensive effort in recent days to reassess the broader war strategy.

Army Gen. Martin E. Dempsey, chairman of the Joint Chiefs of Staff, made an unannounced trip to Afghanistan over the weekend to meet with Marine Gen. John R. Allen, the top U.S. and NATO commander, and other officials.

Dempsey, whose trip wasn’t disclosed publicly until he returned Wednesday, said he wanted to “get a sense for whether our campaign objectives were still valid, whether our campaign plan was still on track.” He said he came away reassured.

“The Taliban is clearly trying to split us apart, but it won’t work,” he told reporters. “They’re working to weaken the coalition, and that won’t work either.”

Dempsey said he kept his Afghan visit under wraps because he originally intended to travel to neighboring Pakistan for a “very private” meeting with Gen. Ashfaq Kayani, that country’s powerful army chief.

He said he wanted to meet with Kayani to discuss another long-standing and sensitive problem that has hamstrung the U.S. war strategy: the existence of cross-border havens in Pakistan for Afghan insurgents. U.S. leaders have criticized the Pakistani military for tolerating and supporting Taliban groups, and Dempsey said he wanted to speak with Kayani “to get his insights and his intentions in that regard.”

But the meeting was postponed, Dempsey said, because of fallout over an anti-Islam video that has sparked protests in Pakistan and other Muslim countries against the United States.