SEOUL (AFP) - South Korea expressed deep concern on Friday after North Korea announced the execution of leader Kim Jong Un's uncle.

In a stunning downfall, Jang Song Thaek - who had been seen as Kim's political regent and the country's unofficial No. 2 - was executed on Thursday immediately after a special military trial, the North's state media said.

"The government has deep concerns about a recent series of developments in North Korea and is watching the situation closely," the South's Unification Ministry said in a statement.