Thai Prime Minister Gen Prayut Chan-o-cha said he did not discuss with his Malaysian counterpart Datuk Seri Najib Razak last week's bomb and arson attacks which killed four people and left scores injured in five provinces.

Prayut was in Malaysia Tuesday to attend the International Conference on National Blue Ocean Strategy in Putrajaya, and the Thai media had speculated that he would raise with Najib the alleged use of a Malaysian mobile phone in the one of the attacks.

The conference, he said, mainly discussed economic and development issues.

"All relevant evidence (on the mobile phone allegedly used in the bombing) has been handed to Malaysia via the security authorities for further investigation," he told reporters when met at Government House here Wednesday.

Reporters asked Prayut whether he had discussed with Najib the alleged use of a Malaysian-registered mobile phone in one of the bombing incidents on Aug 12.

PHOTOS: String of blasts rock Thailand

A security source told Bernama that Thai police had sought the cooperation of their Malaysian counterparts to investigate a mobile phone with a Malaysian serial number that was allegedly used in one of the bombing incidents in Phuket.

The Malaysian police and the Malaysian Communications and Multimedia Commission also confirmed that they were investigating the allegation.

On the ongoing investigation, Prayut advised the public to let the investigators to do their job and urged the media to promote understanding regarding the investigation.

"Please allow the officials to work and find out the wrongdoers. They have a duty to maintain stability in our country that directly affects investment and tourism," he said.

The Prime Minister said he did not want to rule out anything about the possible reasons behind the latest round of violence, including if it was related to the Aug 7 referendum on the draft constitution.

"The authorities are working and there is no goat or sheep (referring to scapegoat) in this case," he said.

Meanwhile, in Hua Hin Wednesday, Deputy Police chief Gen Srivara Rangsibrahmanakul said he had asked the Nakhon Si Thammarat Police chief to withdraw an arrest warrant issued to a 32-year-old Chiang Mai man over an arson charge last week.

Srivara, who headed the investigation on the bomb and arson attacks, said the previous arrest warrant would be replaced with a warrant issued by the military court due to the emergence of new evidence.

The Explosive Ordnance Disposal confirmed that the evidence found at the bombing scene pointed to an explosive device and that it was not an ordinary fire incident, he told reporters.