U.N. Secretary General Kofi Annan, in a rare move, has sent a letter to Prime Minister Junichiro Koizumi to call on Japan to sharply increase aid to Sudan, which is in the midst of a humanitarian crisis.

According to a copy of the two-page letter, dated Monday, Annan said that "much more must be done" to address the crisis in Sudan.

Annan praised Japan's efforts to provide about $2.6 million to the United Nations and its partners and welcomed its intent to consider another $3 million contribution next month.

"However, as a country with a distinguished and most generous record in providing humanitarian assistance, I urge you to consider a sharp increase in Japan's assistance to a level proportional to that provided by some other partners," the letter said.

Attached to the letter was a list of countries and their aid to Sudan from March last year. Of the total aid of about $370 million, the U.S. share amounted to 36.9 percent, Britain 16.4 percent and Japan just 0.9 percent.