U.S. aircraft have flown roughly 4,100 sorties in the air war against Islamic State militants in Iraq and Syria since August, including surveillance flights, refueling runs and bombing raids, a military officer said Monday.

The number of flights reflects the widening U.S.-led air campaign that began with a more narrowly-defined objective when it was launched by Washington on August 8. 

With the world on high alert over foreign fighters joining jihadist ranks in Syria and Iraq, Balkan states are launching efforts to clamp down on recruiting in their region, considered fertile ground by Islamists.

Of the more than 20 million people in southeast Europe, more than five million are Muslims, and an economic slump in weak states battered by past wars has fired up some of the disenfranchised.

Sunni-dominated Saudi Arabia has warned that neighboring Yemen risks sliding towards further violence which could damage regional security, after Shiite rebels overran the capital last week.

Prince Saud al-Faisal, the kingdom's Foreign Minister, called at the United Nations for immediate implementation of a U.N.-brokered peace deal which he said had been flouted by the insurgents, local media reported on Monday.

A Saudi wanted on "terror-related" charges has been killed in a gun battle with security forces in a Shiite village, the Sunni-dominated kingdom's interior ministry said on Sunday.

Basem Ali Mohammed al-Gidaihi "died yesterday (Saturday) afternoon," ministry spokesman General Mansur al-Turki told AFP.

The International Support Group for Lebanon has condemned the hostage taking of Lebanese troops and policemen by jihadists in the northeastern border town of Arsal and reiterated a pledge to strengthen the Lebanese army to confront terrorists.

Participants in the meeting of the Group that was held in New York on Friday said that “the recent attacks by violent extremist groups in Arsal had highlighted immediate priority needs on the part of both the LAF (Lebanese Armed Forces) and the security forces, including in regard to border control and counter-terrorism.”

A series of strikes in Syria overnight by a U.S.-led coalition killed 14 jihadists from the Islamic State group and five civilians, a monitoring group said on Thursday.

The Syrian Observatory for Human Rights said the militants were killed in strikes in the eastern province of Deir Ezzor, and the civilians died in raids in northeastern Hasakeh.

Saudi Foreign Minister Prince Saud al-Faisal, whose country has joined the U.S.-led air strikes against Islamic State group militants in Syria, said Riyadh was ready to help Lebanon if it asked for assistance in its battle against jihadists.

In remarks carried by An Nahar newspaper on Thursday, al-Faisal said: “We have helped Lebanon to buy weapons through a $1 billion donation.”

The United States bombed Islamic State-controlled oil refineries in Syria on Wednesday as President Barack Obama recruited more allies to fight the jihadist "network of death."

U.S., Saudi and Emirati warplanes broadened their bombardment to target the oil installations in eastern Syria that have helped fund the jihadist group's brutal rise from rebel faction to alleged global threat.

By joining U.S.-led strikes in Syria, Gulf Arab monarchies are hoping to eliminate the threat to their authority from Islamic State jihadists and push the regime in Damascus out of power.

But their plan is not without serious risks, analysts warn.

Saudi pilots who conducted air strikes on jihadists in Syria received online death threats Wednesday after photos were published of those involved, among them a son of the crown prince.

The official Saudi Press Agency (SPA) released photographs of eight airmen it said were involved in Tuesday's U.S.-led operation, carried out with Gulf allies.