By JOHN H. CUSHMAN Jr.

WASHINGTON — The weaknesses with the handling of diplomatic security in Benghazi, Libya, that led to a deadly attack on a diplomatic outpost there were “unacceptable,” Deputy Secretary of State William J. Burns told a Senate committee on Thursday.

“We have to do better,” Mr. Burns said in prepared testimony to the Foreign Relations Committee, which opened a hearing into the attack in which four Americans, including the American ambassador, were killed on Sept 11. On Tuesday, one department official resigned and three others were relieved of their duties after a scathing report was released by an inquiry panel led by Thomas R. Pickering, a retired diplomat.

In an opening statement, Senator John Kerry of Massachusetts, the committee’s chairman, said that Congress “also bears some responsibility” to provide adequate financing for diplomatic security. He noted that the board’s report called for spending $2.3 billion a year in the coming decade to protect American embassies and offices abroad.

Both Mr. Kerry and Mr. Burns said it was important to find ways for diplomats to get out among the people, even in dangerous countries.

“We do not want to concertina-wire America off from the world,” Mr. Kerry said.

Mr. Burns said that “diplomacy, by its very nature, must sometimes be practiced in dangerous places.”

“Chris Stevens understood that as well as anyone,” he said, referring to the ambassador to Libya who was killed along with the three others in the Sept. 11 attack. “Chris also knew that every chief of mission has the responsibility to ensure the best possible security and support for our people.”

Mr. Burns and Thomas R. Nides, another deputy secretary of state, told the committee in prepared testimony that the department had “already begun to fix” the “serious, systemic problems” identified in the Pickering report. The two men testified in place of Secretary of State Hillary Rodham Clinton, who is recovering from a concussion.

Mr. Nides said the department accepted “every one” of the report’s 29 specific recommendations. He mentioned, for example, the addition of hundreds of Marines to protect foreign missions. His office is leading an effort to put them into effect “quickly and completely — and to pursue steps above and beyond the board’s report,” he said.

Dozens of specific actions are already under way, several will be completed within weeks, and all will be in motion “by the time the next secretary of state takes office,” he said.

Mr. Kerry is the leading candidate to be replace Mrs. Clinton as secretary of state, following the withdrawal from consideration of Susan E. Rice, the ambassador to the United Nations, after criticisms of statements she made following the attack on the Benghazi outpost.