BAGHDAD - Forty children from northern Iraq's Yazidi minority are reported to have died as a result of a militant attack on the Sinjar region, the United Nations Children's Fund said Tuesday.

"According to official reports received by UNICEF, these children from the Yazidi minority died as a direct consequence of violence, displacement and dehydration over the past two days," a statement said.

On Sunday, fighters from the Islamic State (IS) militant group that controls much of northwestern Iraq took over Sinjar, which had been under the control of Kurdish troops.

The town, near the Syrian border, is a hub for Iraq's Yazidis, a very closed community that follows an ancient faith rooted in Zoroastrianism and referred to by militants as "devil worshippers".

Sinjar was also a temporary home for thousands of displaced people from other minorities, such as Turkmen who had fled the nearby city of Tal Afar when IS launched its offensive on June 9.

The attack on Sinjar sent thousands of people running from their homes in panic, some of them scurrying into the mountains with no supplies.

"Families who fled the area are in immediate need of urgent assistance, including up to 25,000 children who are now stranded in mountains surrounding Sinjar and are in dire need of humanitarian aid, including drinking water and sanitation services," UNICEF said.

Pictures posted on the Internet by members of the Yazidi community show little clusters of people gathering on the cave-dotted flanks of a craggy canyon in the Sinjar mountains.

Yazidi leaders and rights activists have said the very existence of the multi-millennial community on its ancestral land was at risk as a result of the latest violence and displacement.

Meanwhile, the outlawed Kurdistan Workers' Party (PKK), which for years waged a deadly insurgency against Turkish authorities, called on all Kurdish armed groups Tuesday to unite against radical Islamist militants who have seized swathes of northern Iraq.

"Our efforts alone are not enough. There must be a joint (effort)" against the Islamic State (IS), the PKK's military leader Murat Karayilan said in remarks carried by the Radikal online newspaper.

His comments came after IS raised its black flag in the northern Iraqi city of Sinjar on Sunday after ousting the peshmerga troops of the Iraqi Kurdistan government, forcing thousands of people from their homes.

"Let's form a joint command. Let's make preparations and take IS out of the areas it occupied, including Sinjar. This is possible," Karayilan said. Karayilan heads the military wing of the PKK, which has been involved in a now frozen peace process with Turkish authorities, in the absence of the PKK's leader Abdullah Ocalan who is jailed in a Turkish island prison.

The capture of Sinjar and several other towns at the weekend threatened to further integrate the Iraqi and Syrian halves of the "caliphate" IS leader Abu Bakr al-Baghdadi proclaimed in June.

Karayilan urged Kurdish groups to join ranks and form a "national resistance front" to fight against the militants to liberate Sinjar.

He also said peshmerga forces in northern Iraq could play a role. "They are better equipped. We can create a significant force."

On Monday, Baghdad's air force and Kurdish fighters from Syria joined forces with Iraq's embattled peshmerga to push back the militants.

Karayilan singled out main Kurdish parties in Iraq - the Kurdistan Democratic Party and Patriotic Union of Kurdistan - as well as the Kurdish People's Protection Units in Syria as needing to unite against the Islamists.

"We are ready to make every sacrifice... (we) could do this all together. We can support and join it. Our specialised forces could take part," he said.

In a statement on its website, the PKK said Monday none of the Kurds would remain silent to the "humiliating" attacks in Sinjar. "Today is the day for unity and reinforced resistance!"

Meanwhile, Turkish Foreign Minister Ahmet Davutoglu spoke on the phone with the president of Iraq's Kurdistan region Massud Barzani to discuss "the current situation in Iraq."

"We continue our consultations with all parties for stability in Iraq," Davutoglu wrote on his Twitter account.

A Turkish official told AFP that Barzani sought Turkey's support in the fight against the Islamist militants, who have been holding dozens of Turkish citizens including its chief consul in the northern Iraqi city of Mosul since June 11.

Barzani said Iraqi Kurds expected Turkey to help Kurds in the conflict with the IS and called for more consultations with Ankara, the official told AFP on condition of anonymity.

In return, Turkey vowed to support Iraqi Kurds with whom it has forged strong trade links in recent years, according to the official.

Turkey blacklisted IS - known at the time as the Islamic State of Iraq and the Levant (ISIL) - as a terrorist organisation in 2013.