Monrovia (AFP) - A Liberian doctor treated with experimental American anti-Ebola serum ZMapp has died, a minister in the west African nation said on Monday.

Abraham Borbor had been improving but died on Sunday night, Liberian Information Minister Lewis Brown told AFP.

"He was showing signs of progress but he finally died. The government regrets this loss and extends its condolences to the bereaved family," Brown said.

Two other health workers receiving the serum are still in treatment, said the minister, adding that there were "signs of hope".

The World Health Organization (WHO) said on Thursday there had been a "significant improvement" in the condition of a doctor and nurse being treated with ZMapp in Liberia, and that another doctor was getting better, but still in a serious condition.

Liberia took delivery of ZMapp on August 13 from the United States, which gave the serum to two US citizens who were declared cured last week.

The Americans were infected in Liberia along with a Spanish priest who died on August 12, despite also receiving ZMapp.

The very small available stocks of ZMapp, which has never been through clinical trials on humans, have now been used up, according to the lab that produces it.

The WHO has been discussing the use of unapproved drugs as a way of getting a handle on an outbreak in Africa that has already cost more than 1,400 lives.

There is currently no available cure or vaccine for Ebola, but several drugs are under development.

Tokyo said on Monday it was ready to offer an experimental drug developed by a Japanese company to help stem the global tide of Ebola.

Avigan, which is taken in tablet form, was approved as an anti-influenza drug in Japan in March and is currently in clinical tests in the United States.

A panel of medical experts convened by the WHO earlier this month determined it was "ethical" to provide experimental treatments, given the scale of the epidemic.