The Road Transport Department (RTD) has detained four express bus drivers who tested positive for drugs to date, under its drug screening operation since last Tuesday.

RTD director Datuk Seri Ismail Ahmad said, about 622 bus drivers were tested nationwide.

As a result, he said, four drivers were detained - two in Negeri Sembilan and one each in the Federal Territory and Terengganu.

"We have suspended their licences," he told reporters after presenting the RTD Northern Zone Excellent Service Awards here Saturday.

The screening was conducted following the recent spate of accidents involving express bus drivers who had tested positive for drugs.

Ismail said RTD officers have also gone undercover as passengers to spy on offences perpetrated by drivers such as using mobile phones while driving.

He said they would be issued with a summons as soon as they reached their destinations.

RTD officers have also been stationed at the rest-and-recreation areas to check if the first and second drivers carry out the required 300-kilometre change-over from the time of departure, he added.