* Officials say tribes unable to expel militants
* Military to enter city by late Sunday
* Safe passage and amnesty for those who surrender (Adds violence elsewhere in Iraq)
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 are in control of the city, senior security officials and troops told Reuters on Saturday.
Anti-government fighters, among them insurgents linked with al Qaeda, overran two cities in the Sunni-dominated western province of Anbar on Jan. 1. against a backdrop of deteriorating security across the country.
At least 12 people were killed in bombings across Iraq on Saturday, mostly in the capital Baghdad, just 70 km (40 miles) away from Falluja, a city currently surrounded by the army.
Prime Minister Nuri al-Maliki had held off an all out assault on the city to give local tribesmen a chance to expel the militants themselves, but security officials told Reuters a decision had been made to enter Falluja by 6 p.m. (1500 GMT) on Sunday.
"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.
Some of the militants have raised the black banner of the Islamic State of Iraq and the Levant (ISIL), which is seeking to establish a Sunni state straddling the border between Anbar and Syria, where the group is also active.
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."
Maliki has appealed for international support and weapons to fight al Qaeda, although critics say his own policies towards Iraq's Sunni minority are at least partly to blame for reigniting an insurgency.
Once-dominant in Iraq, many Sunnis resent the Shi'ite-led government that came to power after the 2003 U.S.-led invasion.
"Security operations need to go hand-in-hand with inclusive policies, based on the respect for human rights, the rule of law, social development," U.N. envoy to Iraq Nikolay Mladenov said in a statement on Saturday.
Falluja residents and local officials said communications had been cut in the city and its outskirts. Troops stationed in the surrounding area said 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 was deployed on the highway just outside.
More than 140,000 people have fled their homes in Anbar in recent weeks in what the United Nations described as the largest displacement in Iraq since the sectarian slaughter that climaxed in 2006-07.
Officials said airstrikes and shelling on the city would intensify before a ground assault by special operations SWAT teams to clear remaining 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.
A total of 733 civilians were killed in acts of violence across Iraq January excluding Anbar, owing to the difficulty of verifying figures amid the conflict, the United Nations said on Saturday. A further 115 members of the security forces died. (Writing by Suadad al-Salhy and Isabel Coles; Editing by Robin Pomeroy)