The U.S. military dropped 60,000 leaflets on the Islamic State group's de facto capital in Syria showing a grisly depiction of extremist recruits being fed into a meat grinder, the Pentagon said Thursday.

An F-15E fighter jet last week unloaded a canister containing the graphic leaflets on the northern city of Raqa, the latest in an effort by a U.S.-led coalition to undermine the IS extremists' own extensive propaganda and recruiting efforts.

Syrian President Bashar Assad is open to dialogue with the United States, the leader said in an interview with U.S. television network CBS Thursday.

Assad said such dialogue would need to be based on "mutual respect," but that so far there has been no contact from the Americans.

About 440,000 Syrians are now trapped in areas besieged by fighting, more than double the total from last month, as the war reaches "breathtaking levels of savagery", U.N. aid chief Valerie Amos said Thursday.

Amos told the U.N. Security Council that Islamic State fighters are laying siege to government-controlled neighborhoods in Deir Ezzor, northeast Syria, adding 228,000 people to the growing list of civilians in need of aid.

U.N. chief Ban Ki-moon on Thursday noted that Saudi Arabia was carrying out air strikes in Yemen but steered clear of asserting whether the military action was in line with international law.

Ban recalled that the Saudi intervention was at the request of President Abedrabbo Mansour Hadi, who is considered the legitimate head of state by the U.N. Security Council. 

Warplanes from a Saudi-led coalition bombed Huthi Shiite rebels Thursday in support of Yemen's embattled president, who headed to an Arab summit to garner support as Iran warned the intervention was "dangerous."

Defiant rebel leader Abdulmalik al-Huthi slammed the intervention as "unjustified", calling in a televised address for supporters to confront the "criminal oppressive aggression."

The White House on Thursday voiced concern about "Iranian activities" in Yemen, where Huthi Shiite rebels have taken control of several cities and are assaulting the port of Aden.

Alistair Baskey, a National Security Council spokesman, said such actions, as well as "reports of Iranian flow of arms into Yemen," were "contributing to destabilizing the situation and contributing to the threat posed to the legitimate government."

Pakistan's government said Thursday it will dispatch a top civil-military delegation to Saudi Arabia following Riyadh's request that it join a coalition to defend Yemen's president, promising a "strong response" to any threat to the Gulf kingdom.

Prime Minister Nawaz Sharif decided to send the group to Saudi Arabia on Friday after meeting with top defense and military officials in Islamabad late Thursday, his office said in a statement.

Arab foreign ministers meeting in Egypt agreed Thursday on the formation of a joint military force, the Arab League chief and the Egyptian foreign minister said.

The plan forms part of a draft resolution to be submitted to Arab leaders at a weekend summit in Egypt's Red Sea resort of Sharm el-Sheikh.

Turkey could provide "logistical" support for Saudi Arabia's operation against Iran-allied Huthi rebels in Yemen, President Recep Tayyip Erdogan said Thursday, slamming Tehran's involvement in the region.

"We support Saudi Arabia's intervention," Erdogan told France 24 in an interview.

The United States may provide refueling and early-warning radar aircraft support a Saudi Arabian-led operation in Yemen against Huthi rebels, officials told AFP Thursday.

"It's definitely on the table and being discussed," a defense official said of the possible U.S. military assistance to the day-old intervention by an Arab coalition.