"The South Korean authorities do not want their currency to appreciate too rapidly and so they have been intervening, particularly in recent weeks. It doesn't surprise me that there has been a build-up of reserves to reflect that," he added.
The won, trading at around 1,060 to the U.S. dollar on Tuesday, strengthened to around 1,054 last month, its strongest level in more than two years. The currency has been boosted by strong foreign demand for South Korean stocks and bonds, which have gained somewhat of a safe-haven status amid the recent turmoil in emerging markets.
(Read more: Emerging market reprieve is only temporary: Pimco)
"If you look at dollar/won at around 1,050, these levels have marked significant lows for dollar/won over the past few years," said Nizam Idris, managing director, head of strategy,fixed income and currencies at Macquarie Bank.
"The data confirms to us that the BOK bought quite a bit of dollars at that sort of level. It seems like it [1,050] could be a clear line in the sand for the BOK," Idris added.
South Korea's economy is heavily dependent on exports to drive growth. Currency strength, which dents exporter competitiveness, is something that is watched closely.
Indeed, South Korea's Finance Minister Hyun Oh-seok warned last month that recent won strength had been excessive.
Analysts said the jump in foreign exchange reserves also possibly reflected a wish to build up a buffer against any sharp outflow of funds in the event of a pull-back in U.S. monetary stimulus.
(Read more: Taper tease? Market worries Fed will end easing)
Emerging markets have been jolted this year by Fed tapering worries. Still, there has been some calm in recent weeks as investors pushed back their expectations for when the U.S. central bank was likely to start scaling back its $85-billion-a-month asset purchase program.
"I think the BOK is using this as an opportunity to build up a buffer for when taper comes back onto the agenda again and there is pressure on Asian currencies," Idris added.
— By CNBC.com's Dhara Ranasinghe; Follow her on Twitter