A dealer watches computer monitors at a dealing room of Hana Bank in Seoul, Monday. Yonhap

Financial authorities on Monday vowed to take strict action against excessive volatility and one-sided movements in the foreign exchange market amid the weakening Korean won.

"We assess that, in addition to supply and demand factors, certain speculative foreign exchange transactions, such as non-deliverable forwards (NDFs), have recently contributed to increased volatility in the foreign exchange market," a joint statement from the Ministry of Economy and Finance and the Bank of Korea said.

"We will not tolerate excessive volatility relative to economic fundamentals or one-sided market movements, and we will respond firmly to such situations."

The strong verbal intervention came as the Korean won has fallen sharply against the U.S. dollar.