US border agents are monitoring travelers from Japan for signs of radiation, but have found no harmful levels to date, the Customs and Border Protection agency said Thursday.

While stressing that it routinely checks all passengers arriving at US ports and airports, the CBP said it had issued guidance to border staff to pay specific attention to those arriving from Japan.

The agency "is monitoring developments in Japan carefully and is specifically assessing the potential for radiological contamination associated with" the quake and tsunami-crippled Fukushima nuclear power plant, it said.

"Out of an abundance of caution, CBP has issued field guidance reiterating its operational protocols and directing field personnel to specifically monitor maritime and air traffic from Japan," it said in a statement.

"No aircraft entering the United States has tested positive for radiation at harmful levels," it added.

A spokeswoman declined to say if the agency had detected any non-harmful raised levels of radioactivity on passengers arriving since last Friday's devastating earthquake off the coast of northeast Japan.

She also said she was not aware of any specific extra testing being done for passengers from Japan, noting that border staff have a range of devices which test for radioactivity all the time.

These include Personal Radiation Detectors (PRDs), and all airports have more sensitive Radiation Isotope Identification Devices (RIIDs), the CPB statement said.

"Travelers who manifest signs of radiation sickness are referred to health authorities and provided appropriate treatment," it added, stating its general policy.

At seaports and other border facilities agents use sensitive, large-scale Radiation Portal Monitors (RPM) to scan "all maritime cargo and express consignment and mail arriving from Japan," it said.

"CBP will continue to evaluate the potential risks posed by radiation contamination on inbound travelers and cargo and will adjust its detection and response protocols... as developments warrant," it added.