* Officials say tribes unable to expel militants
* Military to enter city by late Sunday
* Safe passage and amnesty for those who surrender
By Suadad al-Salhy
BAGHDAD, Feb 1 (Reuters) - Iraqi security forces are preparing to storm Falluja and break a month-long standoff with militants who control the city, senior security officials and troops told Reuters on Saturday.
Anti-government fighters, among them militants from the al Qaeda-affiliated Islamic State of Iraq and the Levant (ISIL), overran two cities in the Sunni-dominated western province of Anbar bordering Syria on Jan. 1.
Iraqi government forces have since regained control over most of Ramadi, but Falluja remains in militant hands and is surrounded by the army, which has periodically shelled it.
Iraqi Prime Minister Nuri al-Maliki, who is also commander in chief of the armed forces, has held off an assault to give local tribesmen time to expel militants from Falluja themselves.
But security officials told Reuters a decision had now been made to enter the city by 6 p.m. Sunday, Iraqi time.
"That's it: they were given enough time to make their choice, but they failed," a top security official, who spoke on condition of anonymity, said of the tribesmen and militants.
The official said the governor of Anbar had sent a "final warning" to militants and tribal fighters in Falluja. Whoever wanted to leave the city would be given safe passage and those who lay down their arms will be offered amnesty, he added.
"The message was clear, we offered them to leave the city and be a part of the national reconciliation project," the security official said. "But, if anyone insists on fighting our forces, he will be considered an ISIL militant whether he is or not."
Falluja residents and local officials said communications had been cut off in the city and its outskirts. Troops stationed in the surrounding area told Reuters they had received orders to be ready to raid the city.
"We are prepared to enter any minute. Some of our troops in southern and southeastern Falluja have already moved in closer to the city," said a commando whose unit is deployed on the highway just outside.
Officials said airstrikes and shelling would intensify on the city before a ground assault by special operations units known as SWAT, which will clear any pockets of resistance.
"We expect to engage in a fierce battle in the southern areas of the city where militants are holed up," said a SWAT officer deployed in Falluja, who also declined to be named. (Writing by Suadad al-Salhy; Editing by Isabel Coles and Tom Heneghan)