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By Felix Onuah

ABUJA, July 10 (Reuters) - Top U.N. official Ibrahim Gambari has resigned as head of a committee organising peace talks between Nigeria's government and militants in the oil-producing Niger Delta, according to a letter seen by Reuters on Thursday.

Gambari, a former Nigerian foreign minister who is a special adviser to U.N. Secretary General Ban Ki-moon, was selected last month to chair a steering committee charged with preparing the summit in the world's eighth biggest oil exporter.

"My appointment has attracted what appears to be a well of orchestrated and relentless opposition from some quarters, institutions, groups and individuals," Gambari said in the letter to President Umaru Yar'Adua, dated July 9.

"As a result, my name has become the issue in place of the attempt at finding a just and lasting solution to the crisis in the Niger Delta. In light of this, I wish to be excused as the chairman of the steering committee of the summit," it said.

A campaign of sabotage against oil installations in the Niger Delta has cut output from Africa's top producer by around a fifth since early 2006, helping push world oil prices to record highs.

Gambari had said he would hold direct consultations and try to seek a 90-day truce as a first step towards formal talks with militants who have blown up oil pipelines and kidnapped foreign oil workers over the past two years

Gambari was Nigeria's envoy to the United Nations in 1995 when writer Ken Saro-Wiwa and eight other activists were hanged by the then-military government of General Sani Abacha after leading protests against international oil companies.

Some Niger Delta leaders, including the main militant group MEND, rejected Gambari's involvement in the proposed summit, saying he had defended Abacha's actions against international condemnation at the time of Saro-Wiwa's execution. (Writing by Nick Tattersall; Editing by Stephen Weeks)

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