The city of Glendale, about 16 km outside Los Angeles, marked a Korean Comfort Women Day earlier in the week.
"The forced military prostitution by (Imperial) Japan, unprecedented in its cruelty and magnitude, is one of the largest cases of human-trafficking in the 20th century," Mayor Frank Quintero said.
"It is an issue the government of Japan has to address clearly, because it should have been taken care of in the 1940s."
Quintero was speaking at an event organized by a Korean-American group on the fifth anniversary of the House of Representatives passing a bill urging Tokyo to apologize to women forced into sexual slavery by the Imperial Japanese Army.
Kim Bok Dong, 86, who traveled from South Korea to attend annual events in California and Washington on the "comfort women" issue, recounted working in Imperial army brothels during the war and voiced her hopes for an official apology.
"South Korea was freed but I don't think that (comfort women) have been liberated yet . . . we have been gathering in front of the Japanese Embassy for 22 years but still there is no response," Kim said.
"We know we don't have much time left. We aren't doing this for our own benefit, we are doing it for the next generation — such an atrocity should never be repeated.
"I hope the U.S. raises the pressure on Japan, its good friend and ally. To foster a stronger relationship, you need to be able to tell your friend what they did wrong."
The event in Glendale also served as a fundraiser for three comfort women memorials planned in California, which hosts about 500,000 of the 1.7 million Americans of Korean descent.
