The expansion would bring the Cheonghae unit to 395 personnel. Seoul stresses that the mission is non-combat and independent of allied forces.
South Korea's president has ordered the government to consider a powerful central cybersecurity body after a string of breaches at financial and state institutions and a leak of the presidential aircraft's passenger list.
The South Korean military detected launches from the Sondok area. Pyongyang had earlier claimed to have tested a warhead with cluster munitions and a cruise missile with a dispersal warhead.
The missiles were launched from the Wonsan area and flew about 250 kilometres. It is at least the fourth launch in October.
South Korea's Joint Chiefs of Staff and the US are analysing the missile type. The launch took place on Saturday morning towards the East Sea.
President Yoon Suk Yeol declared martial law for the first time since 1980 but was forced to withdraw it after a vote in the National Assembly.
According to the NIS, the first 1,500 special forces have already been moved to Vladivostok on Russian ships. Moscow and Pyongyang have not confirmed the reports.
Pyongyang published the text of the comprehensive strategic partnership treaty signed during the first visit by a Russian president in 24 years. Seoul is considering sending arms to Ukraine.