UNITED NATIONS — A new security agreement authorizing the presence of American forces in Afghanistan after 2014 will be signed just days after the nation’s new president is inaugurated on Monday, a senior State Department official said Wednesday.

Both Ashraf Ghani, Afghanistan’s new president-elect, and his chief opponent, Abdullah Abdullah, indicated during their election campaign that they supported the security agreement. And both men recommitted themselves to the agreement in recent weeks as they worked out the terms of a power-sharing arrangement, American officials said.

“We expect that it will be fully signed in a matter of days after the new administration starts,” said the State Department official, who spoke on condition of anonymity under the agency’s rules for briefing reporters. “No one has talked about reopening the issues.”

Though widely anticipated, the signing of the agreement is an important step that would provide a legal basis for American forces to advise Afghan forces after 2014.