(CNN) -

Staffan de Mistura -- who has been the United Nations' top emissary in Iraq, Afghanistan and other once-volatile locales -- is moving on to Syria, in the hope of turning the tide after years of war there.

U.N. Secretary-General Ban Ki-moon announced Thursday that de Mistura will take on role as the world body's official facilitator on Syria.

That position as the U.N.'s Syria envoy was formerly held by Lakhdar Brahimi and, before that, former Secretary-General Kofi Annan.

Despite considerable, high-level efforts, neither managed to halt a conflict that -- according to the United Nations -- has left more than 150,00 dead and some 680,000 injured since it began in March 2011.

Then there's the humanitarian crisis for the survivors, with the United Nations estimating that more than 9.3 million need help and that at least 6.5 million have been forced from their homes into other parts of Syria. Another 2.5 million are now refugees in neighboring countries.

As Brahimi and Annan found, it's not easy for a U.N. envoy to do anything about this crisis, especially given the dug-in positions of the opposition and the government of President Bashar al-Assad.

Ban expressed confidence that de Mistura's experience working "in very delicate, difficult places like Iraq and Afghanistan" will serve him well in his new post.

"He will bring all his expertise and experience to these very difficult negotiations for peace in Syria, so I am confident that he will be able to carry out his duty as special envoy," the U.N. leader said.

De Mistura, a dual national of Sweden and Italy, began working with the United Nations in the mid-1970s. This career has included stops in Sudan, Ethopia, Vietnam and Somalia.

His international prominence rose significantly with his time in Iraq starting in the 1990s, including as head of a Special Commission charged with dismantling weapons of mass destruction. He also led the U.N. mission in Iraq from 2007 to 2009.

In January 2010, de Mistura was named the U.N. secretary-general's special representative to Afghanistan in the midst of more violence as well as talks involving the Taliban, which has fought against forces loyal to the Kabul-based central government.

British Foreign Secretary William Hague was among those offering "our strong support" to de Mistura in his new effort to foster peace in Syria.

"His appointment comes at a time when the need for a political settlement in Syria has never been more pressing," Hague said.

Reem Salahi, deputy legal director for the National Coalition of Syrian Revolution and Opposition Forces, said her group has been and continues to be willing to negotiate.

That includes working to implement to so-called Geneva Comminique, which calls for a transitional government and eventual free elections as part of a political settlement to end the war -- something that al-Assad's government has not fully embraced.

"We hope that Mr. de Mistura is able to bring the Syrian regime to the table under the terms of the Geneva Comminique," Salahi told CNN. "We are invested in a political transition. We are invested in seeing a democratic state."