Washington's arch-foe Iran has a role to play in tackling Islamic State militants who have overrun large swathes of territory in Iraq and Syria, U.S. Secretary of State John Kerry said.

Iran's supreme leader Ayatollah Ali Khamenei said this week that his government had rejected a request from the United States for cooperation on the battlefield.

Blamed by some for prompting the rise of the Islamic State insurgency, Turkey has finally taken steps to tighten its borders with Iraq and Syria but is still disappointing the West over its low-key role in the anti-IS fight.

Western diplomats accuse President Recep Tayyip Erdogan's Islamic-rooted government of failing to show wholehearted support for the battle against IS militants who control swathes of Iraq and Syria, although Turkish officials bitterly reject these claims.

At least 18 jihadists from the Islamic State group were killed in overnight clashes with Kurdish fighters in northern Syria near a strategic border, a monitoring group said Saturday.

The Britain-based Syrian Observatory for Human Rights said the 18 militants, reportedly including one of Chinese nationality, were killed in fighting near the town of Ain al-Arab, known to the Kurds as Kobane.

President Barack Obama faces growing skepticism in Washington over his plan to defeat Islamic State jihadists in Iraq and Syria without U.S. ground troops, with critics blasting the strategy as unrealistic and half-hearted.

Obama's repeated vow that he will not order "boots on the ground" has infuriated Republicans in Congress, who argue that air power alone cannot roll back the IS extremists given the absence of a viable moderate rebel force in Syria and the Iraqi army's poor track record.

U.S. Secretary of State John Kerry brought together 35 countries at the U.N. Security Council on Friday to build support for Iraq's new government and the campaign to confront jihadists.

The crisis triggered by the "Islamic State" group's seizure of large tracts of Iraq and Syria has challenged countries that are often at loggerheads to confront a common enemy.

Training and equipping moderate Syrian rebels to fight Islamic State extremists will take months, the White House warned Friday a day after Congress approved the plan.

"We will move out as rapidly as can be done, in partnership with the countries that will host the training facilities," said Susan Rice, national security adviser to President Barack Obama.

Prosecutors brought formal charges on Friday against three Germans accused of sending money and camera equipment to help the jihadist group Islamic State make propaganda videos.

The accused were identified only as Karolina M., 25, who has been in pre-trial detention since March, as well as Jennifer Vincenza M. and Ahmed-Sadiq M., both 22, who have not been detained.

British photojournalist John Cantlie, who appeared in a video released by the Islamic State (IS) organization this week, is known as a courageous reporter who has twice been held hostage by the jihadists in Syria.

Some thought he was dead but the 43-year-old re-surfaced on Thursday in a video looking pale, tired and dressed in an orange jumpsuit in an unidentified location.

Denmark said on Friday it would introduce a new, temporary type of residence permit for asylum seekers fleeing civil wars such as the Syrian conflict after a huge surge in the number of applications.

It also unveiled a new plan for stemming the flow of Danish extremist fighters to Syria and Iraq.

People smugglers who allegedly sank a boat killing up to 500 migrants are likely guilty of "mass murder" and should be brought to justice, the U.N.'s new rights chief said on Friday.

"The callous act of deliberately ramming a boat full of hundreds of defenseless people is a crime that must not go unpunished," Zeid Ra'ad Zeid al-Hussein said in a statement.