Skepticism remained over Bank of Japan (BOJ)'s pledge to beat deflation through "quantitative and qualitative monetary easing" as the central bank had little room left for further policy action, a South Korean think tank said Friday.

"Many monetary easing measures that market participants had anticipated were taken at the April policy meeting, reducing room for the BOJ's additional policy action," the Korea Center for International Finance (KCIF) said in a report.

The think tank noted that the reduced room raised possibility for investors to be disappointed at the future policy meeting, boosting skepticism over the BOJ's pledge to get rid of chronic deflation.

The BOJ vowed to achieve 2 percent inflation within two years at the first policy meeting presided over by new governor Haruhiko Kuroda. The bank changed its target for money-market operations from the overnight call rate to the monetary base, or cash in circulation and money that financial institutions have on deposit at the central bank.

The central bank pledged to purchase 7 trillion yen in government debts every month, while lengthening the average remaining maturity of bond holdings to 7 years from 3 years. It will also buy high-risk assets such as exchange-traded funds and real-estate investment trusts worth 1 trillion yen and 30 billion yen every year respectively.

The stronger-than-expected policy action lifted the exchange rate to over 95 yen against US dollar on Thursday. Japanese stocks rallied, and Japanese Government Bond (JGB) yields plunged.

"The market rapidly reacted to the stronger-than-expected monetary easing, but such action entails side effect as excessive monetary easing can threaten fiscal healthiness," said Yoo Hyun-jo, an economist at Shinhan Investment Corp.

Rising inflationary pressure from the monetary easing would lead to a rise in interest rates in Japan, resulting in heavier debt-servicing burden for the Japanese government, with public debts more than doubling its gross domestic product (GDP), Yoo noted.