Kofi Annan is stepping down as UN-Arab League peace envoy for Syria, United Nations chief Ban Ki-moon announced Thursday as battles raged throughout Syria.

After months of shuttle diplomacy, Annan has failed to produce a breakthrough in Syria's 17-month-long crisis. His diplomacy, including a cease-fire brokered in April, has failed to stop fighting between rebels and government forces.

Ban said a search for a replacement has begun. "My consultations with the League of Arab States Secretary-General are under way with a view to the prompt appointment of a successor who can carry on this crucial peacemaking effort," he said. "I remain convinced that yet more bloodshed is not the answer; each day of it will only make the solution more difficult while bringing deeper suffering to the country and greater peril to the region."

The secretary-general admitted his envoy's efforts have not worked. "Both the government and the opposition forces continue to demonstrate their determination to rely on ever-increasing violence," he said. "In addition, the persistent divisions within the Security Council have themselves become an obstacle to diplomacy, making the work of any mediator vastly more difficult."

Syria-government allies Russia and China repeatedly blocked action in the Security Council to stiffen penalties against the Syrian regime. Russia said Thursday that it regrets Annan's decision to quit.

Meanwhile, Syrian rebels have used a captured tank to shell a military base in the northern city of Aleppo, as government forces pounded rebel strongholds in the city with tank and artillery fire.

The opposition Syrian Observatory for Human Rights says the government shelling left seven people dead. The Britain-based group also says cell phone and Internet service is being gradually restored in Aleppo after being cut on Wednesday.

Activists say a security force raid in Damascus on Wednesday left dozens of people dead. Syrian state media said the operation targeted "terrorists," a reference to the rebels.

Another opposition activist group, the Local Coordination Committees, said fighting killed six others in the flashpoint city of Homs. The group also reported warplanes flying over Daraa.

Assad has urged his security forces to "step up the fight" against the rebels trying to push him from power.

A Pentagon spokesman says U.S. Defense Secretary Leon Panetta and Jordan's King Abdullah have discussed what he called the "intolerable acts" of the Syrian regime and the prospects for a political transition in Syria. Panetta is in Jordan as part of a regional tour.