A unit of Citigroup Inc must pay a $15 million fine for not adequately supervising communications among its equity research analysts, clients and the firm's sales and trading staff, Wall Street's industry funded regulator said on Monday.

The supervision lapses at Citigroup Global Markets Inc, which occurred between January 2005 and February 2014, included an instance in which the firm allowed one of its analysts to participate indirectly in two initiatives by the firm to promote clients' initial public offerings to investors, the Financial Industry Regulatory Authority said.

Citigroup, which settled the allegations with FINRA, neither admitted nor denied the charges, but consented to the entry of FINRA's findings.

"We are pleased to have resolved and put this matter behind us," a Citigroup spokeswoman said in a statement. "Citi takes its regulatory compliance obligations seriously, and we believe we have strong procedures and controls in place to address the issues that FINRA has raised in this matter. We are continually working to improve those controls and procedures going forward," the spokeswoman said.

Citigroup issued about 100 internal warnings during the period about communications from equity research analysts, FINRA said. But there were "lengthy delays" before Citigroup disciplined the analysts. What's more, the disciplinary measures were not severe enough to deter more violations of Citigroup's policies, FINRA said.