Intense shelling has rocked neighborhoods in Aleppo, as Syrian security forces stepped up efforts to oust rebels from their positions.

Activists say Syrian President Bashar al-Assad's forces and rebels waged a fierce battle Tuesday for control of the southern part of Aleppo, Syria's largest city. The London-based opposition Syrian Observatory for Human Rights says at least 12 people have been killed.

Meanwhile, Iran says it is holding the United States responsible for the lives of 48 Iranians kidnapped by Syrian rebels Saturday.

State-run media say the foreign ministry told the Swiss envoy in Tehran that it expects the U.S. to use its influence to secure the Iranians' release without any preconditions. Switzerland represents U.S. interests in Iran because Washington and Tehran do not have diplomatic relations. The U.S. has said in the past it is only providing humanitarian assistance to Syrian rebels.

Iran says the 48 abducted in Damascus were religious pilgrims, but the rebel Free Syrian Army described them as Iranian Revolutionary Guards on a “reconnaissance mission.”

Iran's national security council secretary, Saeed Jalili, was meeting Tuesday with Mr. Assad in Damascus. Syrian state television showed the meeting, the first time Mr. Assad has appeared on television in two weeks.

Iran's Foreign Minister Ali Akbar Salehi is visiting Turkey on Tuesday to discuss the Syrian crisis with counterpart Ahmet Davutoglu.

The Turkish Foreign Ministry said Tuesday that more than 1,300 Syrians had crossed the border into Turkey over the past day, raising the total number of Syrian refugees in Turkey to nearly 48,000.