Eyewitness reports suggest Islamic State (Isis) militants have set an oilfield ablaze in the town of Tikrit, eastern Iraq.

Witnesses told Reuters news agency that black smoke had been rising from the oil field from Wednesday afternoon (4 March), as Iraqi security forces and government-aligned militias advanced on the city from the east.

The Ajil oil field is located around 35km (20 miles) east of Tikrit city centre. It was seized by Islamic State fighters last June, when it took over large parts of northern Iraq.

The field had produced around 25,000 barrels of oil per day before Islamic State seized the facility, but output is thought to have declined since then as the militants lack the expertise to operate it at full capacity.

The reports, which were cited by Al-Jazeera and Reuters, come just days after the start of an operation led by the Iraqi army to retake the Islamists' stronghold.

Some 30,000 Iraqi troops and militia, as well as military aircraft, began their biggest and toughest offensive yet on 2 March.

The battle raged between IS fighters and ground troops. Iraqi state-run TV stated on 4 March that government forces were advancing and recapturing oil fields.

Commanders from the Iraqi army were hoping that this operation would be the first of many, and a step towards re-capturing Mosul, the main Isis stronghold in the country.