The International Committee of the Red Cross (ICRC) has expressed concern over a newly-approved US anti-terrorism law.
Mr Kellenberger said the ICRC was still studying the law
ICRC President Jakob Kellenberger said the law raised "questions" about its compliance with the Geneva Conventions on the conduct of war.
He said some points had been omitted, such as the right to a fair trial and the ban on humiliating and degrading treatment of prisoners.
President Bush signed the law on Tuesday, saying it would save US lives.
The Military Commissions Act of 2006 sets standards for the interrogation and prosecution of foreign terror suspects held by the US.
The law aims to enshrine defendants' human rights, but still restricts their right to challenge their detention.
It follows a Supreme Court ruling in June that military tribunals set up to prosecute detainees at Guantanamo Bay violated US and international law.
"Our preliminary reading of the new legislation raises certain concerns and questions," Mr Kellenberger said.
"The very broad definition of who is an 'unlawful enemy combatant' and the fact that there is not an explicit prohibition on the admission of evidence attained by coercion are examples."
Mr Kellenberger said the ICRC would discuss its concern with the White House, such as how the law "omits certain violations from the list of acts that are war crimes under US domestic law".
"These include the prohibition of outrages upon personal dignity, in particular humiliating and degrading treatment, and the prohibition of the denial of the right to a fair trial, which is a basic protection provided for in international law," he said.
Mr Bush said on Tuesday the law would allow the CIA "to continue to question terrorists and save lives".
Mr Bush said the law would save American lives
"It complies with the spirit and letter of the US's international obligations," the president added.
He said the law also set out a system of special tribunals, which would give defendants a fair trial.
"These military commissions will provide a fair trial in which the accused are presumed innocent, have access to an attorney, and can hear all the evidence against them. These military commissions are lawful, they are fair, and they are necessary."
The legislation was passed by both houses of Congress in September after intense debate.
Civil liberties groups say the law does not guarantee detainees' rights, and legal challenges are to be expected.