Heavy fighting continues unabated in Syria's second city Aleppo, as fears grow that the army will launch a full-scale assault within days. Military sources say around 20,000 troops are massed around the city, and rebels said they were prepared for a "strong offensive". In Damascus, army sources said they had pushed rebels from a last stronghold. The rebels said they had withdrawn. On Saturday, 48 Iranian pilgrims were kidnapped near a Shia shrine. Iranian diplomats and Syrian state television blamed the abduction, which took place near the shrine of Sayyida Zainab in a suburb of Damascus, on "armed groups". Rebels claimed on Sunday that some of those taken were members of Iran's elite Revolutionary Guards, according to al-Arabiya television. Fight for Aleppo In the northern city of Aleppo, areas where rebels are entrenched have been bombarded by government forces and clashes have been reported in several districts. President Bashar al-Assad's forces have reportedly used artillery, planes and a helicopter gunship to pound rebel positions. The Syrian military has been steadily building up its forces around the city with, alongside troops, large numbers of tanks and other armoured vehicles, says the BBC's Richard Galpin, on the Turkish border. There is already fierce fighting in and around the city as troops try to push rebel forces out from southern and eastern districts. The rebels, who have also increased their numbers, are well dug in and continue to try to extend the territory under their control, our correspondent says. The biggest advantage for the government is the use of helicopters and fighter jets; but more troops will also have to fight their way into the city if they are to stand any chance of retaking it, and that will make it a much more even battle, he adds. Abdel Jabar Oqaida, a commander of the Free Syrian Army (FSA) in Aleppo, told the AFP news agency that the restive Salah al-Din district had "come under the heaviest bombardment since the battle began" on 20 July. A senior government security official told the agency: "The battle for Aleppo has not yet begun, and what is happening now is just the appetiser... the main course will come later." The fight for the key strategic city has been intensifying over the last few days, with Syrian state television reporting that troops had inflicted huge losses on what it called "terrorist mercenaries" in Salah al-Din and in other nearby areas. 'In government hands' In the capital, government forces claimed to have pushed out rebel fighters from their final stronghold in the city, the southern neighbourhood of Tadamon. An opposition activist told AFP from Beirut that the FSA had withdrawn from the district and would focus on "hit-and-run tactics against important regime targets". On Saturday, shelling and gunfire were still heard in Tadamon despite it having been earlier stormed by government forces. State media has reported that the whole of Damascus is now in government hands, but such reports are impossible to verify and the situation on the ground is changing fast. Referring to the Iranian pilgrims kidnapped in Damascus on Saturday, the Syrian authorities said they were "working to handle the situation". Thousands of Iranians travel each year to Syria to visit the pilgrimage site in the mostly Shia district of Sayyida Zainab, which has seen heavy fighting in recent weeks. In May, 11 Lebanese Shia pilgrims were abducted in Syria while returning from Iran. The government announced three days after their capture that they had been released, but there have since been conflicting reports in Lebanese media as to their whereabouts. Activists say more than 20,000 people - mostly civilians - have died in 17 months of unrest. Meanwhile, US Secretary of State Hillary Clinton will travel to Turkey on 11 August for talks on the conflict in Syria, the US State Department said. Mrs Clinton is adding the stop in Turkey to her lengthy tour of Africa.