WASHINGTON — Afghanistan’s security forces are not yet able to deal with violence in the country on their own, the top American military officer said Wednesday, laying bare one of the biggest challenges facing administration officials as they try to hammer out a peace agreement with the Taliban that could lead to the withdrawal of American troops.

During a rare news conference at the Pentagon, Gen. Joseph F. Dunford Jr., the chairman of the Joint Chiefs of Staff, said he was not ready to use terms like “withdrawal” when discussing the American troop presence in Afghanistan at this point in the peace negotiations.

“Right now, it’s our judgment that the Afghans need support to deal with the level of violence today,” General Dunford said. “If an agreement happens, that could change.”

But he noted: “I’m not using the ‘withdrawal’ word right now. I’m using ‘We’re going to make sure that Afghanistan is not a sanctuary, and we’re going to try to have an effort to bring peace and stability to Afghanistan.’”