SEOUL, Nov 15 (Reuters) - Up to 7 million North Koreans use cell phones daily, and WiFi networks have sharply expanded in recent years as the mobile devices increasingly became a key tool for market activity in the isolated country, U.S. researchers said on Tuesday. Since 3G network services began in 2008, the number of users has risen to 6.5 million to 7 million, more than a quarter of North Korea's 25 million population, the researchers said. The country's antiquated 3G network and limits on foreign investment in upgrades because of sanctions over its weapons programmes has prompted the emergence of faster WiFi networks around the country, Williams said. The WiFi networks do not offer any Internet access but provide connections to domestic services, especially scientific databases for the research community, he added. The private sector has overtaken state-led agents to become North Korea's biggest economic actor in recent years, with its rationing system crumbling and leader Kim Jong Un allowing markets abhorred by his father.

---