U.N. Secretary-General Ban Ki-moon urged countries with the capacity to do so to take decisive action against Islamic State militants, who have taken over large swaths of Iraq and Syria.
"I ... urge the international community, and those with the means, to act decisively and after sober reflection," Ban told reporters Tuesday. "It is critical to keep at the forefront the protection of civilians."
Ban didn't say what steps he hoped U.N. member states would take, but praised recent U.S. air strikes against Islamic State militants, aimed at helping besieged civilians in Iraq.
"These air strikes and military operation which was done at the request of the government of Iraq was able to help the United Nations and other actors to ... save a lot of human lives," he said.
"The United Nations was able to deliver humanitarian assistance to many trapped people in and around Mount Sinjar," Ban said. "It is clear that (Islamic State militants are) a threat to international peace and security, as has already been declared by the Security Council."
U.N. diplomats said that while carrying out air strikes in Iraq didn't raise any legal issues because the Baghdad government has requested it, the legality of bombing militant targets in Syria was problematic.
Many Western states no longer recognize the government of Syrian President Bashar al-Assad as a legitimate authority but diplomats said it was unclear whether the opposition Syrian National Coalition would have the legal authority to invite military action on Syrian territory.
In theory, the Security Council could authorize military action against Islamic State in Syria.
Ban said he expected the 15-nation Security Council to take a "leadership role" in tackling the militant threat, though council diplomats doubt the council would authorize airstrikes on Syrian territory as Russia, an ally of Syria, would likely veto such a proposal.
U.S. President Barack Obama will chair a summit meeting of the U.N. Security Council on Sept. 24 that will adopt a resolution on foreign fighters. Western officials say foreign fighters are among the most savage Islamic State militants, responsible for beheadings, torture and civilian massacres.
At that meeting, the council is expected to adopt a resolution demanding that countries "prevent and suppress" the recruitment and travel of foreign fighters to join extremist militant groups like Islamic State by ensuring it is considered a serious criminal offence under domestic laws.