Accusing the Islamic State in Iraq of murder, hostage-takings and kidnappings, the UN envoy in Iraq on Wednesday called on the Security Council to firmly demand an end to atrocities.

Nickolay Mladenov told the 15-member Council that it was time to take a stand to end to the violence, enforce sanctions to isolate the Islamic State, formerly known as ISIL, and bring perpetrators of war crimes to justice.

"Recruiting and using foreign fighters, engaging in murder, hostage-taking, kidnappings, gross human rights violations, all of which are reasons why the international community and the Security Council should demand, in no uncertain terms, that ISIL cease all hostilities and atrocities," he said.

The top world body must "call upon member states to cooperate in efforts to enforce existing sanctions and hold accountable the perpetrators, organizers and sponsors of these horrific terrorist acts, war crimes and crimes against humanity," said Mladenov by videoconference from Baghdad.

The UN Security Council has denounced the persecution of Christians and urged Iraqi politicians to come together to fight the al-Qaeda offshoot that now controls one third of Iraq including Mosul, the country's second city.

ISIL is on the UN list of terror groups as an al-Qaeda affiliate and is the target of international sanctions.

"The threat of ISIL is not and will not be limited to Iraq. Therefore serious engagement among various stakeholders is urgently needed," said Mladenov, UN Secretary General Ban Ki-moon's special envoy for Iraq.

ISIL fighters launched their advance in Iraq on June 9 and a few weeks later declared a "caliphate" extending from northern Syria to eastern Iraq.

Almost 900 people were killed in July alone, said Mladenov, adding that tens of thousands of Christian and other minorities in Ninewa province had been forced to flee "while many others have been executed or kidnapped."

"Christians have been given an ultimatum to convert, pay a tax, leave or face imminent execution," he said. "Shias, Turkomen, Yazidis and Shabaks are facing systematic abductions, killings or the destruction of their property."

While warning that the situation remained grim, the envoy said the "solution to the crisis cannot be found in the toolbox of military operations."

A new president and government must be elected quickly to lead the country out of crisis, he urged.

"Iraq cannot afford a protracted government formation process, as the current threats continue to challenge the existence of the Iraqi state," he said.