Lakhdar Brahimi, the U.N.-Arab League envoy, was expected to meet Syrian President Bashar Assad on Friday. He also was to meet members of the Syrian opposition.
"We came to Syria to consult with our Syrian brothers," Brahimi said on arrival at the airport in Damascus. "There is a crisis in Syria and I believe it is getting worse."
Brahimi replaces Kofi Annan, who left the job in frustration in August after his efforts failed to stem a conflict that started in March 2011.
The visit comes as violence convulses the country's largest city, Aleppo, and the outskirts of the capital, Damascus. Activists said regime forces shelled Aleppo and clashes with rebels were reported outside Damascus.
The two cities were once seen as largely immune to the violence in other parts of Syria, but have been hit by fighting as rebels try to bring the fight to symbols of Assad's power.
Although the regime is better armed than the rebels, the government has not been able to crush the rebellion. The rebels also have failed to overthrow the regime, leading to a bloody stalemate that many fear will drag on indefinitely.
The violence has left the Assad government isolated internationally, although Iran, China and Russia support it. Brahimi met Mohammad Riza Shibani, the Iranian ambassador to Syria, on Thursday—a meeting the ambassador described as "good and fruitful."
He also met Syrian Foreign Minister Walid al-Moallem, who stressed that any initiative should "focus on the Syrian people's interest," said the state-run news agency, SANA.
In Baghdad, U.K. Foreign Secretary William Hague said Assad's regime is "doomed" and should not be allowed to survive after committing crimes against its people. He said a transition of power is the only way forward.
"That is the only way to avoid protracted civil war, or the collapse of the Syrian state, or an even greater flow of refugees and loss of life," Hague said at a joint news conference in Baghdad with his Iraqi counterpart, Hoshyar Zebari.
AP writer Sinan Salaheddin contributed to this report from Baghdad.