The International Committee of the Red Cross (ICRC) expressed concern on Thursday at the United States' tough new anti-terrorism law.
The president of Swiss-run humanitarian body, Jakob Kellenberger, said that there were questions over its compliance with the Geneva Conventions on the conduct of war.
The Military Commissions Act of 2006, signed by President Bush on Tuesday, allows for tough CIA interrogation techniques and military trials for terrorism suspects.
But in an interview posted on the ICRC website, Kellenberger said the law was too vague about which detainees could be covered and did not explicitly exclude the use of evidence extracted by force in trials.
He warned that the legislation could also weaken basic guarantees given under the Geneva Conventions which are supposed to protect everybody from humiliating and degrading treatment.
"Our preliminary reading of the new legislation raises certain concerns and questions," 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" he added.
According to the AP news agency, officials at the US mission in Geneva said on Thursday evening they were unable to comment as they had yet to study Kellenberger's comments.
The ICRC, guardian of the Geneva Conventions which are accepted by 194 nations, said it was studying the law, which it said was very complex and had both positive and negative elements.
Kellenberger said the organisation would discuss its concerns as part of its ongoing dialogue with the US.
President Bush says the new law will enable the US to bring to trial some of those believed to be behind the September 11, 2001 terrorism attacks on the country.
The legislation eliminates some of the rights defendants are usually guaranteed under US law and authorises the continued harsh interrogation of terror suspects, a provision Bush says is vital. However, the law also aims to enshrine defendants' human rights.
It follows a Supreme Court ruling in June that military tribunals set up to try detainees at the US military installation at Guantanamo Bay, Cuba – where the ICRC has been carrying out visits - violated US and international law.
Under the new law, the definition of "enemy combatant" is expanded to include those who provide weapons, money and other support to terrorist groups, which human rights groups say casts the net too wide.
They also attack provisions they say strip foreign suspects of the right to challenge their detention in US courts.
swissinfo with agencies
The controversial Military Commissions Act of 2006 was signed by President Bush on Tuesday.
President Bush says the law will enable the United States to bring to trial some of those believed to be behind the September 11, 2001 terrorist attacks.
It follows a US Supreme Court ruling in June that military tribunals set up to try detainees at the US military installation at Guantanamo Bay, Cuba, violated US and international law.
The ICRC has been visiting detainees at Guantanamo Bay regularly. Last week it confirmed that it had for the first time visited terrorism suspects recently transferred to the detention centre.