TOKYO (AFP) - Officials in the Japanese city of Yokohama were left red-faced on Wednesday after mistakenly announcing the launch of a North Korean missile to 40,000 followers on Twitter.

The city, south of Tokyo, prematurely fired its Tweet just before noon, announcing "North Korea has launched a missile" with blank spaces to indicate the exact time.

"We received a call from one of our followers who had noticed the mistake.

We had the Tweet ready and waiting, but for an unknown reason it was dispatched erroneously," said a city official.

The city retracted the Tweet about 20 minutes later and apologised to followers of @yokohama-saigai, the official said.

Japan is on full alert ahead of an expected mid-range missile launch by North Korea, with Patriot missiles stationed in its capital to protect the 30 million people who live there.

In addition to PAC-3 batteries, Aegis destroyers equipped with sea-based interceptor missiles have been deployed in the Sea of Japan (East Sea).

Twitter is wildly popular in Japan, where the ideograph and pictograph-based language is well-suited to the 140 character limit, allowing much more information to be imparted than alphabetic languages like English.