Fierce fighting is being reported for a second straight day in the Syrian city of Aleppo, as government troops and rebel forces battle for control of the country's commercial capital.
Helicopter gunships and artillery shells were used by fighters loyal to President Bashar al-Assad to launch attacks on neighborhoods under control of rebel fighters.
Activists have expressed fear that many civilians in the city of 2.5 million may be injured or killed in the violence.
International peace envoy Kofi Annan has called on both sides to show restraint and work in favor of a political solution to the 16-month conflict.
The battle for Aleppo comes as Syrian Foreign Minister Walid al-Muallem arrived in Tehran for unscheduled talks with Iranian officials on the issue.
Meanwhile, Russia warned that a "tragedy" was looming in Aleppo, a city of more than 2 million people near the border with Turkey.
Russian Foreign Minister Sergei Lavrov told journalists it was unrealistic to expect the government would stand by when armed rebels were occupying major cities.
Based on reporting by Reuters, AFP, and AP