Hong Kong has widened its top-level black travel alert to three more Japanese prefectures after explosions at a nuclear plant in the quake-stricken country deepened concerns of a meltdown.

The warning, announced late Tuesday, advises Hong Kong citizens to avoid travel to affected areas amid rising concerns about dangerous radiation seeping from the stricken facility about 250 kilometres (155 miles) northeast of Tokyo.

"In view of the seriousness and uncertainty of the incident of the nuclear power plants in Fukushima Prefecture in Japan, the government has extended the Black Outbound Travel Alert from one prefecture to four prefectures," a government statement said.

The alert advises citizens to avoid all travel to Miyagi, Ibaraki and Iwate Prefectures, which are near Fukushima where the plant is located.

A black alert was issued for Fukushima on Saturday. The rest of Japan including Tokyo is under a red alert, one level below black, which advises against non-essential travel.

The government said special arrangements have been made for Chinese citizens to leave Japan, without elaborating. The Standard newspaper in Hong Kong reported that buses have been arranged to transport Hong Kong residents to local airports.

Hong Kong has no immediate plans to scan inbound travellers from Japan for radiation contamination because the Japanese government is already conducting tests for those who had been within 20 kilometres of the plant, the city's Undersecretary for Security Lai Tung-kwok said Tuesday.

After explosions and fires at the atomic facility this week, Japanese officials said radiation near the plant had reached levels harmful to human health.

Tens of thousands have already been evacuated from a zone within a radius of 20 kilometres from the Fukushima No.1 plant.

On Wednesday, some Hong Kong lawmakers questioned why the black travel alert was not extended to other areas of Japan.

"This is illogical and unreasonable -- Tokyo is also an affected area, why is the black travel alert not raised for the city?" asked James To, chairman of the city's Legislative Council security panel.

But security chief Ambrose Lee insisted the alert levels were "right", saying "we based our decision on advice from radiation experts and the potential impact of the radiation on humans".

Meanwhile, the Hong Kong Observatory said Wednesday it was unlikely the southern Chinese financial hub was at risk.

"Based on the Observatory's assessment of the weather situation, the chance of radiation from Japan reaching Hong Kong in the next few days is slim," it said in a statement.

"The path of the air mass approaching Hong Kong in the coming days clearly indicates that its origin is far from Japan," it added.