Mayor Bill de Blasio today tore into Gov. Andrew Cuomo’s reported agreement with the Republican-run State Senate to repeal parts of a major gun control bill passed in 2012—arguing that the recent shooting of black churchgoers in Charleston, S.C. proved the need for strong regulations on firearms.

Speaking to reporters after an unrelated event in the Bronx, the mayor attacked a memo his fellow Democrat issued last week that the Republicans claim strip controls on ammunition sales from the Secure Ammunition and Firearms Enforcement, or SAFE Act. The law was passed in 2013 in the aftermath of the mass shooting of elementary school students in Newtown, Conn., and Mr. de Blasio argued the murders in South Carolina last month reinforced the urgent necessity of tough gun regulations.

“I can say this much: that law was passed by the legislature, signed by the governor, it was the right thing to do and we just can’t go backwards,” Mr. de Blasio said, though he admitted he was not familiar with the nuances of the new arrangement. “Charleston I think was another wake-up all for this country. And we just cannot go backwards on gun safety. That’s the bottom line.”

According to press releases by upstate State Senators Catherine Young and James Seward, the memo eliminates the act’s call for an as-yet uncreated state database tracking all ammunition sales, which would facilitate background checks for people purchasing bullets. The memo itself says that the state will not allocate any resources to the creation of such a database until the superintendent of the State Police, who answers directly to the governor, presents a design for it.

The agreement appears to be an attempt to appease rural Republicans who disliked the law, which was passed against their protests two years ago, and who distrusted the Long Island-based Mr. Flanagan, who supported the measure at the time.

Mr. Flanagan replaced fellow Long Island Republican Dean Skelos—another SAFE Act backer—as State Senate majority leader in May after Mr. Skelos had to step down amid federal corruption charges. Mr. Flanagan reportedly enjoyed the support of both Mr. Skelos and Mr. Cuomo in his bid for the powerful perch, which pro-gun State Senator John DeFrancisco of Syracuse also sought.

Mr. Cuomo’s office claimed that the memo is essentially meaningless, and amounts to a simple clarification of the current state of the database, which the governor’s staff said is still in the works.

“No provision of the SAFE Act has been rolled back or altered due to this memorandum. As the Governor’s Counsel said, this is merely a restatement of fact and plans to move forward with the database continue,” said spokesman Rich Azzopardi.