North Korea's Air Koryo sent five planes to Russia's Vladivostok on Monday local time, and it's not yet clear why.
The Vladivostock airport has confirmed they were 3 passenger planes and 2 cargo planes.
It's been reported that two of the passenger planes and one of the cargo planes have already gone back the North.
Normally, there are two regular passenger flights between Pyongyang and Vladivostok each week.
It's rare to see more flights than that.
There's speculation that these planes could have been carrying North Korean officials to meetings on expanding bilateral trade and investment,... going to pick up North Korean workers in Russia to bring them back... or preparing for a visit to Russia by Kim Jong-un.