Russia has said a military jet's intrusion into South Korean territorial airspace was not intentional, according to Seoul's presidential office.
A Russian military official expressed "deep regret" to the South Korean defence ministry and blamed a technical glitch, South Korea said.
On Tuesday, Seoul said a Russian warplane had twice violated its territorial airspace.
The Russian defence ministry had earlier furiously denied this.
But South Korea's Blue House said in a briefing that Russia had now said the violation was unintended and that it would immediately launch an investigation into the case.
"Moscow said if the aircraft flew according to an initially planned route, this incident would not have occurred," a spokesman told reporters.
This has not been confirmed by Russian officials.
Also on Wednesday, China's defence ministry denied any of the planes had entered the territorial air space of any country.
Russia's defence ministry had said its plane was taking part in a joint air patrol by Russian and Chinese warplanes over the Sea of Japan and the East China Sea - the first ever air patrol between the two countries.