TOKYO - Toyota Motor Corp will suspend operations at all of its assembly plants in Japan from Tuesday afternoon due to a malfunction with its production system, it said, likely bringing domestic output to a standstill for the world's top automaker.

Toyota was looking into the cause of the problem, a spokesperson said, adding it was "likely not due to a cyberattack". The malfunction has meant it has not been able to order components, the spokesperson added.

Toyota suspended operations at 12 of its plants from Tuesday morning, with two remaining online. All 14 will be suspended from the second shift on Tuesday, the spokesperson said, adding the amount of lost output was unclear.

Altogether the 14 plants account for around a third of Toyota's global production, according to Reuters calculations. Toyota is the world's largest automaker by sales.

The suspension comes as Toyota's Japanese production had been on the rebound after a series of reduced output plans it had blamed on semiconductor shortages. Production in Japan was up 29% in the first half of the year, the first such increase in two years. ​ Daily production in Japan for Toyota brands, excluding Daihatsu and Hino, averaged about 13,500 vehicles in the first half, based on a Reuters calculation of working days and excluding holidays.

Toyota's operations ground to a halt last year when one of its suppliers was hit by a cyberattack. That one-day disruption caused an output loss of around 13,000 cars.

The automaker is a pioneer of the just-in-time inventory management, which keeps down costs but also means that a snarl in the logistics chain can put production at risk.

While the cause of the malfunction was not clear, corporate Japan has been on alert in recent days as some businesses and government offices reported a flood of harassing phone calls.

The government has said the calls were likely from China and related to Japan's release of treated radioactive water from the Fukushima nuclear power plant into the Pacific.

Shares of Toyota were down 0.3% at 2,429 yen in early Tokyo trade.

(Reporting by Satoshi Sugiyama; additional reporting by Miyoung Kim and Kevin Krolicki; Editing by David Dolan and Christopher Cushing)