"Regular dose information is now being received from 47 Japanese cities," Graham Andrew, scientific and technical advisor to the head of the International Atomic Energy Agency, told a regular daily press briefing here.
"Dose rates in Tokyo and other cities remain far from levels which would require action. In other words they are not dangerous to human health," he said.
The IAEA has just sent a radiation monitoring team to Japan to help authorities determine whether any dangerous radiation has been released from the stricken Fukushima nuclear plant, 250 kilometres northeast of Tokyo.
"First measurements in Tokyo showed no indication of iodine 131 or cesium 137. These are fission products which come from the fuel," Andrew said.
The team would now move to the Fukushima region "as soon as possible" to continue monitoring there.
The IAEA said important data about radiation levels by a fellow UN agency that monitors for clandestine nuclear tests appeared to back up its assessment that current radiation readings posed no threat to human health.
Vienna-based diplomats with access to that data from the Preparatory Commission for the Comprehensive Nuclear-Test-Ban Treaty Organization (CTBTO) said radiation had reached Sacramento in southern California in the US, but first readings were well below levels that would be harmful to health.
"I am aware that we have the data, but I haven't studied it and I don't think my colleagues have either," Andrew said.
But it appeared to "confirm what I said earlier." After the release of radiation from Fukushima, there have been calls for radioactivity readings from the specialist agency's network to be made available.
The CTBTO has calculated the course of the plume of radiation over the Pacific to the United States and other countries.
However, the body is normally only allowed to share the data with member states and not directly with the public.
Nevertheless, it confirmed in a statement that it would share its monitoring data and analysis reports with the IAEA and the World Health Organisation.