Syrian government forces are reported using tanks and artillery against rebels in the major city of Aleppo, a day after the rebels held off a government offensive.

The Britain-based Syrian Observatory for Human Rights says intense fighting is taking place Sunday in southwestern districts of the city.

The battle for control of Aleppo, Syria's commercial hub, is crucial for both the government and opposition.

The opposition Syrian National Council is urging the U.N. Security Council to hold an emergency session on Aleppo, alleging that the government is planning to commit a massacre there.

As the fighting raged, Syria's Foreign Minister Walid al-Mualem made a previously unannounced visit to Syria's staunch ally, Iran.

At a Tehran news conference, he asserted that what he called the “anti-Syrian forces” gathered in Aleppo “will definitely be defeated.”

Iran's Foreign Minister Ali Akbar Salehi warned about the consequences for the region and the world should the Assad government fall. He said it is “naive” and “an illusion” to think that if there is a power vacuum in Syria and a transition of power, a new government could easily be established.

Meanwhile, Jordan on Sunday opened its first official tent camp to help host Syrians who have fled their country.

Foreign Minister Nasser Judah says Jordan is now sheltering more than 142,000 Syrians. He says up to 2,000 more are arriving each day.

Turkey is sheltering more than 44,000 Syrians.

The United Nations says more than 17,000 people have been killed during Syria's conflict since March of last year.