Sporadic clashes broke out on Monday near Dhuluiyah as security forces and allied tribesmen prepared for an operation against militants who have repeatedly attacked the Iraqi town, officials said.
Dhuluiyah, north of Baghdad, was previously overrun by militants, but local Sunni tribes and police drove them out, and residents have put up fierce resistance to renewed attacks.
The area would appear to be the target of the next major drive against jihadist-led militants who have overrun large areas of Iraq, after a successful operation to break the siege of the Turkmen Shiite town of Amerli farther north.
A police major and Ahmed al-Krayim, head of the Salaheddin provincial council, who was in Dhuluiyah, both reported clashes.
They also said a mortar round apparently containing poison gas hit the town, causing breathing problems but no deaths, accounts confirmed by a doctor at the local hospital.
Dozens of tribal leaders met in a Baghdad hotel on Monday for a conference also attended by former prime minister Nuri al-Maliki and meant to drum up support for more decisive military action against Islamic State militants in Dhuluiyah.
"The coming hours will be critical for Dhuluiyah," said Sheikh Riyadh Abdallah al-Juburi, a senior Sunni tribal leader from the southern part of Dhuluiyah which has resisted IS from the start.
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"If we don't continue to hold our ground, the consequences would be disastrous... but defeating Daash (IS) in Dhuluiyah would be a turning point for the whole of Iraq," he told AFP during the conference.
Dhuluiyah was one of the main purveyors of Sunni tribal fighters who helped turn the tide on gains by IS's previous incarnation in 2005-2007, with backing and funding from the US occupation forces.
Juburi said he had called on the foreign powers providing military assistance to the Iraqi federal and Kurdish forces to support a major operation against Dhuluiyah.
"I have already asked for that and it should materialise soon," he said, without elaborating.
Krayim said reinforcements have already been sent to the area and efforts are under way to repair a bridge across the Tigris River that was bombed by militants, so more can be brought in.
Resident Abu Abdullah said people have turned to crossing the Tigris by boat to buy food, but even that is unsafe -- a mortar round targeted boats on Sunday, wounding many people, and snipers are also a threat.
Militants launched a major attack on Dhuluiyah on September 8 using gunmen and two suicide bombers.
One bomber detonated a vehicle to breach a barrier around a southern neighbourhood while the second struck inside. Eighteen people were killed, but the attack was repulsed.