PARIS — U.N. special envoy Lakhdar Brahimi will visit Syria soon to try to persuade Bashar al-Assad's government to call an immediate cease-fire in an 18-month-old conflict with rebels, U.N. Secretary General Ban Ki-moon said on Tuesday.

Efforts by Brahimi's predecessor, Kofi Annan, to engineer a truce collapsed within days, with neither the Damascus government nor opposition forces willing to abide by conditions for an effective cessation of hostilities.

Brahimi is to meet Assad as fighting rages in Syria's biggest city Aleppo and government forces pursue offensives to dislodge rebels from provincial bastions elsewhere, causing increasing spillover into neighboring countries especially Turkey, prompting Ban to warn against the danger of escalation.

"Brahimi is now going to the region again and he will visit several countries and after that he will visit Syria," Ban told a news conference along with French President Francois Hollande after the two met in Paris.

Ban said Brahimi aimed to curb the bloodshed and negotiate a deal to allow more humanitarian aid into Syria, where a civilian protest movement has evolved into an armed insurgency and 1 million people have been driven from their homes.

"First and foremost, the violence must be stopped as soon as possible," Ban said. Diplomats said Brahimi would first visit Saudi Arabia, Turkey and Egypt, all regional diplomatic heavyweights, for consultations before heading to Damascus.

Brahimi's spokesman Ahmad Fawzi later clarified that Brahimi's initial swing through other countries meant his trip to Syria would not happen this week. He declined to give details of Brahimi's itinerary for security reasons.

In September, his first month on the job, Brahimi met Assad in Damascus and visited Syrian refugee camps in Turkey and Jordan. The U.N. envoy said afterwards that he had a "few ideas" but no full plan on how to defuse the conflict, which he described as "extremely bad and getting worse."

On Monday, Turkish President Abdullah Gul warned that "worst case scenarios" were playing out in Syria as Turkey's army fired shells over the border for the sixth day running in response to shelling from the Syrian side. Northern Syria near the Turkish border has seen heavy fighting in the civil war.

LEERY OF UNILATERAL TRUCE

Asked how Assad reacted to calls for a cease-fire, Ban said he had conveyed a "strong message" for a unilateral truce.

"Of course, their reaction was what will happen if they do it and the opposition forces continue [to fight]?" he said.

Ban said he was discussing how to provide assurances to both rebels and the government in talks with the U.N. Security Council and countries in the region. "I am getting positive support from the key countries," he said.

He repeated a call for those countries providing weapons to both sides to stop. Saudi Arabia, Qatar, and Turkey have backed the rebels, while Assad's main allies are Iran and Russia.

Turkey has bolstered its military presence along the 900-km (560-mile) border with Syria and responded in kind to gunfire and shelling coming from the south, where Assad's forces have been battling insurgents holding swathes of territory.

Hollande, among the most outspoken Western critics of Assad, said he would push for more punitive sanctions against Damascus in hope of forcing the Syrian leader to the negotiating table.

"The difficulty we are facing is not linked to the U.S. election, but to the division at the U.N. Security Council to take immediate decisions that would be useful to the Syrian people," he said.

Russia and China have vetoed Western-backed attempts to have the Council pass harsh U.N. sanctions aimed at isolating Assad.

Activists say more than 30,000 people have been killed in the uprising against Assad.