Apartment prices in Seoul have fallen 4.7 percent on average compared to a year ago, Kookmin Bank said Thursday. It was the highest on-year drop since February 1999 in the wake of the Asian financial crisis.

Apartment prices in the swish Gangnam, Seocho and Songpa districts of southern Seoul, the most expensive in the city, fell more than 6 percent, the biggest drop in the capital. Those three districts have traditionally been favored locations for real estate speculation, so prices there tend to fluctuate wildly according to the economic situation.

On-month, apartment prices fell 0.4 percent in January, marking 21 consecutive months of decline since May 2011 and the longest since statistics began in 1986.

"January is the slow season, and tax exemption for real estate acquisition ended at the end of last year, so apartment prices dropped amid plunging real estate sales," a Kookmin Bank staffer said.

According to a website run by the Seoul Metropolitan Government, there were fewer than 1,200 apartment sales last month, the lowest level since 2006.