The Syrian army continued its attacks on rebel positions Sunday in the northern city of Aleppo with heavy shelling and helicopter gunships.

The country's biggest city has become a key battleground in the nearly year and a half uprising against President Bashar al-Assad's rule, as government forces reportedly mass ahead of what is expected to be a strong offensive against the opposition.

The opposition Syrian Observatory for Human Rights reported that more than 40 Syrians, including 25 civilians, were verified as killed Sunday in the country. The casualties have not been independently verified.

Meanwhile, Iranian media said Tehran has asked Turkey and Qatar to help secure the release of 48 Iranian nationals kidnapped Saturday in Damascus. Iran says the victims were religious pilgrims, but a brigade commander with the Free Syrian Army describes them as elite Iranian Revolutionary Guards.

Iran backs the Syrian government, while Turkey and Qatar support the Syrian opposition.

U.S. Secretary of State Hillary Clinton is expected to visit Turkey next Saturday to discuss the deteriorating situation in Syria.

A State Department official made the announcement Sunday as Clinton visited Malawi during her African tour.