GENEVA (Reuters) -- He struck a unique deal with Syrian President Bashar Assad, fought what he viewed as excesses in the US "war on terror", compiled reports that exposed US mistreatment of detainees in Iraq, and fought for inmates' rights at Guantanamo.

Jakob Kellenberger, who describes himself as "the negotiator of last resort," steps down on Friday after 12 years as president of the International Committee of the Red Cross.

The independent aid agency's 12,500 staff perform frontline humanitarian operations, delivering food and medical care in war zones from Syria to Somalia.

Kellenberger's tenure was marked by conflicts triggered by the Sept 11, 2001 attacks and US efforts to capture or kill al-Qaida militants abroad, including by drone attacks. It ends with the prospect of Syria descending into full-blown civil war.

"When I look back, it was a very central issue, the whole issue of what the (President George) Bush administration called the global war on terror," Kellenberger, 67, said in an interview in his office in the ICRC's Geneva headquarters.

"The basic balance between military necessity and humanity and respect for human dignity was broken in some places and in other places it was in serious danger. I had really the feeling that you had to fight against this from the start," he said.

Noting that many of his files relate to fighting in Afghanistan and the Middle East, he added: "It's almost a bit symbolic that it ends with this region, with Syria, but before it was Lebanon, Israel, the (Palestinian) territories."

The Bush administration, which invaded Afghanistan in late 2001 and Iraq in March 2003, openly questioned the relevance of the Geneva Conventions to al-Qaida and Taliban fighters, a challenge to the ICRC which is the guardian of the 1949 rules of war that aim to protect civilians and prisoners of war.

In Afghanistan, the ICRC quickly gained access to visit US-held detainees in Kandahar and Bagram, and to those sent to the US naval base in Guantanamo, Cuba, without being charged.

Its confidential reports on US mistreatment of detainees in Iraq, including at Abu Ghraib, appeared in the Wall Street Journal in 2004, sparking an international outcry.

"It was very difficult for us, it was a typical dilemma of confidential issues. What came out after in photos, what became public, was in fact already part of the ICRC reports from visits to Abu Ghraib to the end of 2003 but were confidential reports, for US authorities and nobody else," he said.

Confidential access

Worldwide, the ICRC visited more than 540,000 detainees last year in 75 countries.

They included Saif al-Islam Gaddafi, son of the late Libyan leader Muammar Gadhafi who was captured on the run in the desert, and former Ivory Coast President Laurent Gbagbo.

The only international agency to deliver assistance in Syria, the ICRC has visited prisons in Damascus and Aleppo. It is still seeking access to all detainees held by both sides.

Kellenberger has gone to Syria three times in the past nine months to negotiate better access, and has held two meetings with Assad. The ICRC has said that local civil wars have raged in Hama, Homs and Idlib, but that there is not yet an overall civil war, a view some politicians feel is too qualified.

"I'm really afraid that political efforts will be too slow ... I am afraid that we go down the way to a country-wide internal armed conflict," he said.

On Tuesday, aid workers from the ICRC and Syrian Red Crescent were on their way to Homs to evacuate trapped civilians and wounded from the old city, but negotiations were still under way to secure safe access.

That task now falls to Kellenberger's successor, Peter Maurer, another senior Swiss diplomat, who takes over on July 1.

The ICRC's internal rules stipulate that its interviews with inmates -- meant to evaluate their treatment and conditions of detention in a bid to prevent torture -- are private. In exchange, the agency only shares its findings with the detaining authorities.

"I do continue to strongly believe that in the long-run confidentiality as a tool remains extremely important to have access to detainees but also to have access to certain regions," Kellenberger said.

"But I have also to admit it is clear that the publication of what happened in Abu Ghraib had large effects, in the good sense. Because I mean in some quarters of the American authorities they became very much aware that there were things to be changed."

Kellenberger, who held talks with George W. Bush and Donald Rumsfeld on Guantanamo and other issues, said that Washington had been open to discussions, even to "very critical dialogue".

Despite "deep divergences of view on legal and protection matters" the United States never threatened to pull the plug on the ICRC, to which it is a major donor, he said.

Two months ago, Kellenberger held talks with President Barack Obama about concerns over US detainees and other issues.

"That was a very positive meeting. In fact I said to him that I felt very grateful that they were always so supportive even if I was aware that I was a very difficult guy.

"Then he told me, 'Please remain difficult'," he recalled with a laugh.