U.N. chief Ban Ki-moon urged Somalia's al-Qaida-linked insurgents to end violence during a surprise visit Friday to war-torn Mogadishu, where he arrived wearing a bullet-proof jacket.

Flanked by a guard wearing a white U.N. helmet, Ban was welcomed at the airport by Prime Minister Abdiweli Mohamed Ali on the first visit by a U.N. chief to Mogadishu, often described as the world's most dangerous city, in almost two decades.

Turkey may consider setting up a buffer zone on its border in coordination with the international community in the event of a massive exodus of refugees from Syria, its foreign minister said on Tuesday.

"We are preparing for every scenario," Foreign Minister Ahmet Davutoglu told Kanal 24 television channel when asked whether Turkey was mulling the creation of a buffer zone.

A new Arab League ultimatum which gives Syria until later Friday to agree to accept observers or face sanctions represents a last chance for the regime in Damascus, Turkey's foreign minister said.

"It is a last chance, a new chance for Syria," Ahmet Davutoglu told reporters in Istanbul in the final hours before the 11:00 GMT deadline.

Syria has reached a "point of no return" in the regime's crackdown on opposition demonstrators, Turkish President Abdullah Gul said on Wednesday during his state visit to Britain.

"Unfortunately Syria has come to a point of no return," Gul said in a speech, adding that the whole region could be dragged into "turmoil and bloodshed" by the crisis.

Turkish Prime Minister Recep Tayyip Erdogan on Tuesday called on his former ally Syrian leader Bashar Assad to step down.

"Quit power before more blood is shed ... for the peace of your people, your region and your country," Erdogan said in parliament.

Turkish Prime Minister Recep Tayyip Erdogan warned Syrian President Bashar Assad Monday that his days as leader were numbered and he cannot remain in power indefinitely through military force.

After reports that two buses carrying Turkish pilgrims came under attack in Syria, Erdogan said his one-time ally's defiant refusal to end a bloody crackdown on protesters had increased the prospects of foreign intervention.

French Foreign Minister Alain Juppe said on Friday that the "time has come to increase sanctions" against the Syrian regime.

"The situation is no longer sustainable," and "the ongoing repression is unacceptable," Juppe told reporters during a visit to Ankara.

Rebel troops hit offices of Syria's ruling party on Thursday, a day after a daring raid on an intelligence base that prompted Russia to warn that its longtime ally risks "full-scale civil war."

The rocket-propelled grenade attack in northwestern Idlib province, near Turkey, came as security forces killed eight people, including two children, despite an Arab League ultimatum that Syria halt the bloodshed or risk sanctions.

Syrian President Bashar al-Assad's regime will be isolated by the Arab world if it does not halt the deadly crackdown on its own people, Turkish Foreign Minister Ahmet Davutoglu said Wednesday.

"The cost for the Syrian administration of not fulfilling the promises it made to the Arab League is its isolation in the Arab world as well," Davutoglu said at a meeting with Arab League foreign ministers.

Turkey has abandoned hope that Syria will respond to international demands to halt violence and initiate democratic reforms, Prime Minister Recep Tayyip Erdogan said Tuesday.

"We no longer expect the Assad government to show honest, persuasive, brave and determined leadership," Erdogan told his party's lawmakers. "No one expects him to respond to the demands of the international community anymore."