Gates did not say whether he planned to pursue charges against Wikileaks, the organisation that published the documents on its website and handed them over to media organisations.
Admiral Mike Mullen, the chairman of the joint chiefs of staff, lashed out at Wikileaks and its founder, Julian Assange. He said the leak endangered the lives of Nato soldiers, and of civilians and Afghan citizens working with Nato.
"They might already have on their hands the blood of some young soldier or an Afghan family," Mullen said.
Assange said the organisation withheld 15,000 documents as part of a "harm minimisation process".
But The Times reported on Thursday that a cursory search of the leaked documents "found the names of dozens of Afghans credited with handing intelligence to US forces".
Hamid Karzai, the Afghan president, also expressed concern for the safety of Afghans named in the documents.
At a press conference in Kabul on Thursday, he called the leaks "shocking" and "irresponsible".
"Their lives will be in danger now," he said. "This is a very serious issue."
At a press conference earlier this week, Assange said Wikileaks had a "backlog" of other sensitive material.
US officials have speculated that the organisation may have more classified documents about the wars in Iraq and Afghanistan.
"The reality is, at this point, we don't know how many more there are out there," Gates said. "It could be a substantial additional number of documents."