Nuclear uncertainty and the chance of more tremors have prompted Switzerland to advise its citizens to leave northeastern Japan and the Tokyo conurbation if possible.

The Swiss president, Micheline Calmy-Rey, told journalists on Wednesday after a special cabinet meeting that for the moment people could return home on commercial flights.

But she added that in case of need, charters would be arranged.

The situation at the Fukushima nuclear power plant in northern Japan is still not under control after it was damaged by Friday’s earthquake and tsunami. Calmy-Rey noted that “a worsening of the situation is possible”.

She said the Swiss Federal Nuclear Safety Inspectorate was following developments in Japan and keeping the government informed.

About 1,890 Swiss are registered with the embassy in Tokyo, which has been able to contact 1,592 of them. There has been no news of any Swiss casualties, she said.

The embassy is supplying iodine pills to Swiss citizens in Japan in case they are exposed to radiation. Such pills would prevent the thyroid gland from absorbing radioactive iodine 131 which could be released by a nuclear accident.

Meanwhile, Swiss Post has been refusing to accept letters and parcels for Japan since Monday. It explained that planes are having to carry enough fuel for the return flight as well and there is no room for extra cargo.

A Post spokesman said aid items would be given priority on flights.

However, the backlog had been sent via an alternative route on Wednesday, and the post office would  know on Friday if this was an efficient alternative.

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