South Korea has sent a letter to a sanctions panel under the U.N. Security Council to call for an "appropriate response" to North Korea's launch of two ballistic missiles earlier this week, a diplomatic source said Friday.

"Our government's letter was sent to the UNSC sanctions on Thursday (New York time)," a government source said on the condition of anonymity.

The communist state fired the missiles into the East Sea on Thursday morning in an apparent protest against the ongoing military drills between South Korea and the United States, which it denounced as a rehearsal for a northward invasion.

The Seoul government was expected to call for a probe into the North's latest provocation and stress that it was a violation of UNSC resolutions banning any launch using ballistic missile technology.

The U.S. is also expected to send a similar letter to the sanctions panel, the source said.

Sending a letter to the panel is considered to be a relatively lenient response to the North's latest provocation. But it is intended to call international attention to the North's violation of UNSC resolutions and is expected to help justify tougher sanctions should Pyongyang engage in another provocation in the future.

After the sanctions panel receives the letter, a group of experts will initiate an investigation into the North's provocation, the outcome of which will later be reported to the UNSC, Seoul officials explained.

In May, Seoul sent a letter to the panel to ask for an investigation into Pyongyang's ejection test of a submarine-launched ballistic missile. It also sent letters to the panel after the North fired a series of short-range ballistic missiles on several occasions in 2014. (Yonhap)