More than 1.3 million internet-connected devices around the world, including personal video recorders and security cameras, have been infected with computer viruses, according to a recent study by a group of Japanese researchers.

The devices concerned, which form part of the so-called internet of things, are believed to be controlled by cybercriminals and used in denial of service attacks targeting businesses, according to a lab led by associate professor Katsunari Yoshioka at the graduate school of Yokohama National University.

Many such devices, including TVs, digital cameras, automobiles and aircraft, are poorly defended against cyberattacks, and about 1,000 devices in Japan are known to be infected with viruses, according to the lab.

"Manufacturers should confirm the security (of connected devices) before shipping them," Yoshioka said.

Yoshioka has been monitoring cyberattacks against his university's communication network since last spring. By tracking IP addresses, he detected attacks from 30,000 such devices around the world in January alone.

The number of attacks began to surge this summer, exceeding the 1 million mark in September and reaching 1.33 million in October, due partly to the massive spread of a virus called Mirai that targets connected devices. Some devices were found to be spreading viruses after being infected, according to the research team.

The cyberattacks against Yoshioka's university originated mainly from devices in Vietnam, China and Brazil. The lab identified the use of more than 500 types of devices including fire alarms and building control systems. In Japan it also found infections in home-use routers and solar-power generation systems.

Experts say some types of viruses could also cause infected devices to malfunction.