BANGKOK (AFP) - Thailand's prime minister appealed for calm on Tuesday (Oct 11) after police warned of a plot to target Bangkok with car bombs, sparking a security alert across the capital, including at airports.

An unusually detailed police memo was handed to reporters on Monday (Oct 10) warning that an unidentified group was planning to target Bangkok between Oct 25-30.

The memo said "areas such as malls, carparks and tourist attractions" were at risk and ordered police to be extra-vigilant.

In the last year, Thailand has been rocked by blasts hitting its crucial tourist sector, a rare bright spot in an otherwise lacklustre economy.

The junta has refused to label the assaults terrorist attacks and has played down suggestions tourists are being deliberately targeted.

"Let officials carry out their jobs and please be confident in their work," Prime Minister Prayut Chan-o-cha, who seized power in 2014, told reporters.

He said the the bomb plot was an "ongoing warning" and an investigation was underway, but warned people not to panic.

For more than a decade, Thailand has had a notoriously turbulent domestic political scene and a festering Muslim Malay insurgency in its far south.

But until recently, foreigners had largely avoided being caught up in the violence. That appears to be changing.

In early August, coordinated blasts struck multiple places in Thailand's south, killing four Thais and wounding dozens, including foreign visitors.

A year earlier, a bomb tore through a Bangkok shrine popular with Chinese visitors, killing 20 and wounding more than 100. Most of the victims were ethnic Chinese overseas visitors.

Both attacks were followed by confusing and contradictory statements from authorities, with military leaders initially keen to blame domestic political opponents.

But as investigations progressed, clearer motives emerged.

Police believe the multiple bomb attacks in August this year were carried out by Muslims from the "Deep South".

Shadowy militants have waged a 12-year fight there for more autonomy, but rarely attack targets outside of the three southern Malay-speaking provinces.

Monday's police memo did not say who might be behind the bomb plot, though the southern insurgents have used car bombs in previous attacks.

Oct 25 is also the anniversary of 2004's "Tak Bai incident", when more than 80 people in the deep south died, most of them protesters crushed to death in overloaded vans after they were arrested.

The deaths lit the fuse of the current southern insurgency.

According to Thai police, last year's shrine bombing was the work of two Chinese Uighurs who are currently on trial for the attack.

Most analysts believe the bombing was revenge for Thailand's forcible deportation of 109 Uighurs back to China, where rights groups say they face significant repression.

Thai authorities maintain the attack was not political and was carried out by a passport forgery gang angry at a policing crackdown.