Lakhdar Brahimi, a veteran diplomat troubleshooter, will take over from Kofi Annan as the international envoy on the Syria conflict, the United Nations said.
UN leader Ban Ki-moon appealed to the divided international powers to give "strong, clear and unified" support to the new envoy, who in turn said he was not confident he could end the 17-month-old civil war.
Annan is stepping down at the end of the month. The former UN secretary general complained about the lack of international support shown for his six-month campaign to make President Bashar al-Assad and opposition fighters end their hostilities.
The announcement came Friday as the Syrian army clashed with rebels near the main military airport in Damascus and shelled southern parts of the capital as well as areas of the commercial city of Aleppo and the eastern province of Deir Ezzor, human rights observers said.
Deadly violence was also reported in the provinces of Homs and Daraa, the cradle of the uprising that began with peaceful protests in March 2011 but has escalated into an increasingly vicious battle between armed rebels and government forces.
Brahimi, 78, has vast experience handling conflict-stricken states.
He was Algeria's foreign minister from 1991-93 and later became a UN envoy in Afghanistan before and after the September 11, 2001 attacks, and in Iraq after the 2003 invasion.
While representing the Arab League, Brahimi helped end the Lebanese civil war in the 1980s, negotiating with the Syrian government of the time.
UN deputy spokesman Eduardo del Buey said Brahimi would come to New York "soon" for talks.
"The violence and the suffering in Syria must come to an end," Ban said in a statement released by his spokesman.
"The secretary general appreciates Mr Brahimi's willingness to bring his considerable talents and experience to this crucial task for which he will need, and rightly expects, the strong, clear and unified support of the international community, including the Security Council," the spokesman added.
Russia, Assad's main ally, and China have vetoed three UN Security Council resolutions on Syria, accusing Western nations of only seeking forced regime change.
Brahimi has been in prolonged talks with Ban on the role and at times there has been doubt over whether he would take the post.
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Seeking to distinguish himself from Annan, Brahimi will be known as the joint special representative instead of joint special envoy.
But he will still act for the United Nations and the Arab League, even though Assad's government has refused to recognize Arab League involvement in the mediation.
Brahimi said again that the UN Security Council must overcome its bitter divisions on Syria, where activists say more than 23,000 people have died.
"We are going to discuss very, very seriously how they can help," Brahimi told the France 24 news channel. "They are asking me to do this job. If they don't support me, there is no job so I am looking forward to discussing with them."
Asked whether he was confident the civil war could be ended, Brahimi said: "No, I'm not. What I am confident of is that I am going to try my utmost, my very, very best."
Diplomats said Brahimi had sought a sign of "strong support" from the Security Council before accepting.
In a letter to Ban on Thursday, the 15 Security Council members said they "reiterated their support to your good offices and to the mission of the joint special envoy for Syria."
But in a new sign of the international divisions over the conflict, Western and Arab nations boycotted a meeting called by Russia in New York. Russia postponed the meeting.
Russia's UN envoy Vitaly Churkin said his government had wanted to discuss a possible appeal to Assad and the opposition to end the fighting.
But the United States, Britain, France and Arab nations Qatar and Turkey told Russia they would not attend, diplomats said.
Only China and the United Nations accepted the invitation, while Iraq was the only Arab nation to express an interest.
US Secretary of State Hillary Clinton endorsed Brahimi as the new international envoy, while the White House asked for the UN to provide answers about his mandate.
Britain said it "fully supports" Brahimi's appointment, welcoming the "vast experience" he brought to the role of seeking a political solution to the violence.