A South Korean expert says the ballistic missile that North Korea fired on Sunday was likely launched from an underground facility, not a mobile launch pad.

The expert says photos released by North Korean media show two pillars of smoke billowing into the sky in a V-shape -- a special feature of underground launches.

The expert, speaking on condition of anonymity, says Pyongyang may try to test-fire larger missiles, including the ICBM-class Hwasong-17, from underground silos.

In November last year, North Korean media reported that the country's leader Kim Jong Un had mentioned the preparation of an underground launch pad in connection with the development of the Hwasong-17.

Missiles launched from underground silos are said to be more difficult to detect.

Analysts say North Korea apparently wants to show it has various methods for carrying out missile launches.