Two of the funds in the city's $155 billion pension system dropped their holdings in gun manufacturers such as Smith & Wesson Holding Corp. and Sturm Ruger & Co. Inc. after the Sandy Hook school shooting in 2012.

Those two funds were the New York City Employees Retirement System and the New York City Teachers Retirement System. Funds for the city's police and fire departments and the city's board of education have not divested.

"I call on all government pension funds in New York City and across the country to divest immediately from funds that include assault weapon manufacturers," de Blasio said in a statement.

De Blasio also appealed to private investors to dump gun stocks and funds that invest in them.

Wednesday's massacre in San Bernardino, in which 14 people were killed, was the worst U.S. mass shooting since 26 children and adults were shot dead at an elementary school in Sandy Hook, Connecticut, in December 2012.

The mayor urged the city comptroller "to divest as soon as possible if no verifiable assurance is given that assault weapons will not be sold to civilians."

The comptroller's office, which oversees the funds, said it was down to the mayor to present detailed plans to pension fund board members.

"We look forward to receiving that proposal," said John McKay, a spokesman for the comptroller. "Gun violence is a real and constant threat to our children, families and communities."

Investments in gun makers across the three funds amounted to $2.1 million, as of Sept. 30, according to a person with knowledge of the assets. The person asked not to be named as the funds do not publish detailed asset lists. The three funds had assets of $46 billion as of that date, the person said.

The comptroller's office could not immediately say which gun stocks are held by the funds.

New York City's pension system is complex. The five funds each have separate boards staffed by around 50 board members, including city officials and labor union representatives.

The Patrolmen's Benevolent Association, New York City's largest police union, did not return a request for comment. The Uniformed Fire Officers Association and trustees for the Board of Education Retirement System could not be reached.