Iraqi forces halted a short-lived offensive yesterday to recapture Tikrit, home town of executed dictator Saddam Hussein, due to fierce resistance from Islamic State fighters who have also threatened to attack Americans “in any place”.

In Geneva, the United Nations refugee agency announced a major aid operation to get supplies to more than half a million people displaced by fighting in northern Iraq.

Buoyed by an operation to recapture a strategic dam from the jihadists after two months of setbacks, Iraqi army units backed by Shi’ite militias launched their offensive shortly after dawn on Tikrit, a city 130 kilometres north of Baghdad, which is a stronghold of the Sunni Muslim minority.

But officers in the Iraqi forces’s operations room said by mid-afternoon that the advance had stopped.

South of Tikrit, the government side came under heavy machinegun and mortar fire from the militants, a group of Arab and foreign fighters hardened by battle both in Iraq and over the border in Syria’s civil war, the officers told Reuters.

To the west, landmines and snipers frustrated efforts to get closer to the city centre in the latest in a series of attempts to drive out the militants.

Residents of central Tikrit said by telephone that Islamic State fighters were firmly in control of their positions and patrolling the main streets.

Sunni Muslim fighters led by the Islamic State swept through much of northern and western Iraq in June, capturing the Sunni cities of Tikrit and Mosul as well as the Mosul dam, a fragile structure which controls water and power supplies to millions of people down the Tigris river valley.

Analysts believe Assad – who is firmly in control in the capital more than three years into the civil war – is seizing the moment to show his potential value to Western states that backed the uprising against him but are now increasingly concerned by the Islamic State threat.

Islamic State’s recruitment drive

Thousands of fighters joined Islamic State in Syria last month in its fastest expansion yet, a war monitoring body said yesterday.

Now in control of roughly a third of Syria and large areas of Iraq, Islamic State has been seizing territory from rival Islamist groups in a belt of territory north of Aleppo, threatening rebel supply lines into the city where President Bashar al-Assad’s forces are seeking to encircle the insurgents.

Islamic State recruited at least 6,300 men in July, Rami Abdelrahman, founder of the Syrian Observatory for Human Rights, told Reuters – a big expansion from early estimates suggesting the group numbered around 15,000. Around a thousand of the new fighters were foreign, and the rest Syrian, he said.