A cabinet statement said a voluntary recruitment drive was necessary to fill shortages in squads in the west of Anbar province.
Thousands have fled Ramadi since its capture by IS on Sunday.
Meanwhile, the US National Security Council said it was considering "how best to support local ground forces".
Spokesman Alistair Baskey told AFP that some of the measures may include "accelerating the training and equipping of local tribes and supporting an Iraqi-led operation to retake Ramadi".
A more detailed announcement could come within days.
Stand united
Pro-government forces and police officials told Reuters news agency they had thwarted an IS attack to the east of Ramadi overnight, near to where Shia militias have been deployed ahead of a counter-offensive.
Witnesses earlier reported seeing Islamic State militants setting up defensive positions in the city itself.
After a Council of Ministers meeting on Tuesday, the Iraqi prime minister vowed to prosecute forces who fled the city in the wake of the IS attacks.
Mr Abadi said the Iraqi people needed to "stand unified" and called for voluntary recruitment to the army. He also pledged to recruit and arm tribal fighters.
The council also issued a fresh plea to the international community to help Iraq's "war against terrorism".