Hong Kong on Tuesday raised Outbound Travel Alert for Thailand (Bangkok) to red, after a bomb exploded in the center of Bangkok last night killing four Chinese including two Hong Kong women.

"Residents intending to visit Thailand (Bangkok) should adjust their travel plans and avoid non-essential travel, including leisure travel. Those already there should pay attention to announcements by local authorities, attend to their personal safety and avoid areas affected by the explosion incident," said a government spokesman.

The Outbound Travel Alert for other parts of Thailand remains at amber.

Also on Tuesday morning, Chief Executive CY Leung expressed his condolences for people who died in the Bangkok explosion, and wished the injured a speedy recovery.

Leung urged the Thai government to carry out a thorough investigation into the incident, while appealing for Hong Kong people there to pay attention to their personal safety.

Doctors and immigration officers will be sent to provide assistance and support, he added.