KILIS, Turkey — More than 1,300 Syrians fled to Turkey overnight to escape growing violence in their country as rebels try to expand their hold inside Syria’s largest city despite two weeks of withering counterattacks by President Bashar Assad’s troops.

Heavily armed government troops have been steadily shelling rebel-controlled parts of the city, particularly Salaheddine and other districts on the southwestern edge of Aleppo as the two sides fight for control over the strategic city.

Aleppo-based activists said clashes were going on today near the historic city center in Bab Jnein and Sabee Bahrat districts, suggesting the rebels were making some inroads. They said intense bombardment of the town of Tal Rafaat near the Turkish border sent scores of people spilling into Turkey for safety.

A Turkish government official said 1,328 Syrian refugees had crossed the border by midmorning — nearly double the number of refugees who reached Turkey on Monday.

The new arrivals would increase the number of Syrians who have found refuge in Turkey, which has served as a staging ground for rebels fighting Assad’s regime, to close to 48,000.

“We are expecting a massacre in Aleppo. The regime is bringing reinforcements to the city because they consider that if Aleppo falls, the regime will fall,” said a Syrian refugee in Turkey who identified himself as Abu Ahmad.

“The city is being bombed from the air and ground,” he said, adding he was in daily contact with residents still in the city.

The rebels appeared also to be bringing in reinforcements.

A video posted online by activists today showed a large group of Free Syrian Army rebels in military fatigues and carrying shoulder carried rocket propelled grenades and automatic rifles, announcing they were joining the “Unification Brigade,” the main group of rebel fighters in Aleppo, to assist in the “liberation” of the city.

“They have Satan on their side, we have God on ours,” one rebel shouts. “We are coming, Aleppo,” shouts another. The authenticity of the video could not be independently verified.

Despite a ferocious crackdown, rebels in Syria have grown more confident and are using increasingly bolder tactics both in Aleppo and the Syrian capital, Damascus.

In a brazen, daylight attack, rebels on Saturday abducted a group of 48 Iranians near Damascus, branding them as spies assisting in Assad’s crackdown against the rebels.

Iran said those captured when their bus was commandeered were pilgrims visiting a Shiite shrine on the outskirts of Damascus. Today, Iran’s Foreign Ministry said it holds the U.S. responsible for the fate of the abducted Iranians.

Iran’s state IRNA news agency said the ministry summoned the Swiss envoy in Tehran late Monday to stress that Iran expects Washington to use its influence to secure the Iranians’ release.