FALLUJA, Iraq, April 11 (Reuters) - Insurgents in Iraq have added water to their arsenal of weapons after seizing control of a dam in the west of the country that enables them to flood certain areas and prevent security forces from advancing against them.
The dam helps distribute water from the Euphrates river on its course through the western province of Anbar, and is located some 5 km south of the city of Falluja, which was overrun by militants early this year.
Iraqi troops have since been surrounding Falluja and shelling the city in an effort to dislodge anti-government tribes and insurgent factions including the Islamic State in Iraq and the Levant (ISIL).
In February, ISIL took control of the Nuaimiya area where the dam is located, and began fortifying their positions with concrete blast walls and sand bags, according to anti-government tribesmen who said no other groups were involved in the takeover.
The militants closed all eight of the dam's 10 gates one week ago, flooding land upstream and reducing water levels in Iraq's southern provinces, through which the Euphrates flows before emptying into the Gulf.
Anti-government tribal fighters said ISIL's tactic was to flood the area around the city to force troops to retreat and lift the siege on Falluja.
"Using water as a weapon in a fight to make people thirsty is a heinous crime," said Oun Dhiyab, a government adviser to the water ministry. "Closing the dam and messing with Euphrates water will have dire consequences."
By Thursday, militants had re-opened five of the dam's gates to relieve some pressure, fearing their strategy would backfire by flooding their own stronghold of Falluja, some 70 km (44 miles) west of Baghdad.
Iraqi security officials said flooding around the city had already forced many families to leave their homes and prevented troops from deploying or operating properly there in order to stop militants encroaching on the capital.
"They (ISIL) want to use the flood waters to make it difficult for the security forces to deploy in those areas and this is their chance to move the battle outside Falluja," said an anti-government tribal leader inside the city.
The Falluja dam is also key to a number of irrigation projects in the desert province of Anbar, which shares a border with Syria.
In his weekly televised address, Prime Minister Nouri al-Maliki, who held off a full-on ground assault on Falluja, vowed to exact revenge from the militants for interfering with the water supply.
"The murderers took advantage of the government policy of utmost restraint in Falluja ... But it seems the situation has become more complicated and necessitates confrontation," Maliki said.
Two army officers in Ramadi and Falluja said preparations were underway to launch a quick attack to regain control of the Falluja dam.
"We are carrying out aerial surveillance to spot militant positions near the dam," said one army officer whose regiment received orders to prepare for mobilising from Taji, to the north of Baghdad, to Falluja.
"A military operation could start very soon".
Iraq is a patchwork of desert and arable land. Its inhabitable areas are fed by the Tigris from Turkey, the Euphrates from Turkey and Syria, and a network of smaller rivers from Iran.
The decline of water levels in the Euphrates has also led to electricity shortages in towns south of Baghdad, which rely on steam-powered generators that depend entirely on water levels.
A spokesman for the Ministry of Electricity said the power supply from Mussayab power station had decreased to 90 megawatts from 170 megawatts.
Government officials and advisers warned that ongoing closure of the dam could affect irrigation of farms in many southern provinces that depend on the Euphrates, including Hilla, Kerbala, Najaf and Diwaniya.
"Iraq is close to national elections and it seems they want to force the government into a corner," said one senior security official on condition of anonymity. (Reporting by Ahmed Rasheed and Raheem Salman; Writing by Isabel Coles; Editing by Catherine Evans)