The United States warned that foreigners could face kidnapping risks in the coastal areas of Sabah.

"There are indications of continued planning of kidnappings, including of foreigners, in eastern Sabah's coastal areas," the State Department said in a "public announcement".

It said that several kidnappings and piracy incidents had occurred in this area, most recently in March.

The Abu Sayyaf kidnap-for-ransom group, based in the southern Philippines, was responsible for some of these incidents, while the others were attributed to unidentified criminals, it added.

"Because of this, American citizens should exercise caution when visiting the coastal areas of eastern Sabah," the advisory said.

It also cautioned Americans traveling overland from Malaysia to Thailand to exercise special caution while in the far south of Thailand.

Last month, the State Department urged American citizens to defer non-emergency travel in that area as a result of recent unrest in the region.

On April 3 and 4, a series of bombs were detonated in Hatyai and nearby Songkhla city in Songkhla province, and in Yala province in Thailand's far south. Two American citizens were injured in the most recent attacks.