The Islamic Resistance Movement (Hamas) issued a statement received by Aljazeera condemning what it described as "the US aggression against Najaf".
"The US occupation forces must withdraw from Najaf and the whole country (Iraq)" said the statement.
"Withdrawal from Iraq is the first step in ending Iraq's disaster."
The Lebanese Shia Muslim authority Grand Ayat Allah Muhammad Hussain Fadhl Allah, has warned of dangerous consequences, if the US forces occupied the city of Najaf.
In a statement issued by his office, Fadl Allah has called for the expulsion of the occupation from the country by all means.
The Arab League expressed deep concerns at the deteriorating security situation in Iraq, describing the events in Najaf as an indication of instability in the war torn country.
In an exclusive interview with Aljazeera, Arab League secretary-general Amr Mousa said an Iraqi national conference is the only solution to end hostilities in Iraq.
"An Iraqi-Iraqi dialogue must start, even if it was violent at the beginnig, it does not matter; as long as its final end is national reconciliation" he said.
The Association of Muslim Scholars, the highest Sunni Muslim authority in Iraq issued a fatwa saying it is prohibited for any Muslim to aid the occupation troops to fight his Muslim brothers.
The association issued a statement warning Iraqi police and National Guards they will be hated, if they continue to side with occupiers against their brothers in Najaf.
The International Committee of the Red Cross issued a statement expressing concerns over the increasing number of Iraqi casualties in the ongoing fighting between Iraqi fighters and US forces in Iraq.
The statement urged the warring parties to secure safe passage for the injured people to reach hospital.
The committee said it is ready to provide urgently needed assistance.