Syrian government troops clashed with rebels for a second straight day in the northern city of Aleppo, as the intensifying conflict sent thousands of civilians from across the war-torn nation pouring into neighboring Lebanon and Iraq.

Activists described the fighting in Aleppo as some of the fiercest to date in Syria's commercial hub and largest population center. The city has remained largely loyal to President Bashar al-Assad and been spared the daily bloodshed that has plagued other areas.

Residents said Damascus was largely calm Saturday after days of heavy clashes that left corpses on the streets of the capital.

The United Nations reported that an estimated 30,000 Syrians have crossed into Lebanon since Thursday. Authorities in Iraq say about 1,000 people arrived from Damascus by air, while thousands more people trying to escape the escalating violence arrived from Syria by land.

Turkey has registered more than 43,000 refugees from Syria, but many others are believed to have crossed into Turkish territory without being counted.

Rebel fighters in Syria either have control of or are attempting seize several official border crossings. Fighters from the Free Syrian Army continue to control the main Bab al-Hawa crossing into Turkey, which Syrian government forces abandoned under fire.

An attempt by Western and Arab nations to impose U.N. Security Council sanctions on Syria failed this week after Russia and China vetoed the plan. The council Friday approved a final 30-day extension of the monitoring mission in Syria, with the understanding this would allow only enough time to schedule an orderly withdrawal of U.N. personnel now in the war zone.

U.N. Secretary-General Ban Ki-moon said Saturday he has dispatched his peacekeeping chief, Herve Ladsous, and top military adviser, Senegalese General Babacar Gaye, to Syria to assess the situation. Gaye will replace Norwegian Major General Robert Mood as head of the observer mission.

The 300 unarmed military observers had been sent to Syria earlier this year as part of U.N. envoy Kofi Annan's unsuccessful effort to end the conflict. Human-rights estimated the violence in Syria has killed at least 17,000 people over the past 16 months.