North Korean hackers have stolen data from nearly 20% of the smartphones operated by the South Korean spy agency. The hack has eventually helped them get hold of more confidential information of top South Korean authorities.

Though the exact extent of the penetration is not yet known, Seoul's National Intelligence Service (NIS) admitted malicious software has infected one in five smartphones of their operatives. The hackers have gained access to the contents of the devices, including text messages, contacts and other conversations.

Stopping short of revealing the names of the South Korea officials affected by the hack, NIS said senior government figures were among those impacted by the incident. Pyongyang has not yet responded to the accusations.

The incident, which is said to have taken place between late February and early March, is the latest in a series of cyber attacks emerging from the North in recent months.

Seoul's state-run NIS is holding an emergency meeting today (8 March) with the relevant South Korean ministries in order to address the situation. The incident came to light just a day after South Korean President Park Geun-hye called on government officials to brace for possible cyber attacks from the North. She also urged the parliament to adopt a bill to effectively prevent cyber threats from Pyongyang.

In addition to the development, Seoul has also unilaterally clamped sanctions against Pyongyang on top of international measures. Lee Suk-joon, minister for government policy coordination, said: "First, we are expanding the scope of financial sanctions related to North Korea. Second, we are strengthening maritime regulations," adding the North's import-export operations will be severely curtailed.

Pyongyang, which has thousands of trained expert hackers, has been blamed for several previous cyber attacks on Seoul.