Unusually heavy monsoon rains have flooded Sri Lanka's capital and neighbouring districts where mudslides killed at least 13 people, officials said Monday.

The island's western and southern regions experienced heavy rains with more than 100 millimetres (four inches) dumped within about one hour on Sunday, an official at the Disaster Management Centre (DMC) said.

"The weather appears to be getting worse and we could have more rain in the coming days," DMC director Sarath Lal Kumara told AFP.

He said a search was underway for two people missing after a mudslide in a Colombo suburb on Sunday night. Police and DMC records showed 13 people had been killed since Sunday evening while at least another two were reported missing.

Train services were disrupted after lightning destroyed a signalling system while electricity was cut off in vast areas after trees brought down power lines.

The monsoon hits the country twice a year, bringing vital rain for irrigation but also causing frequent loss of life and damage to property.

At least 54 people, mostly fishermen, were killed in cyclonic winds that accompanied the monsoon in June last year.