AMMAN- Syrian troops fought rebels trying to seize central Aleppo on Tuesday and quelled a jail mutiny on the outskirts of the northern city, killing 15 prisoners, opposition activists said. After a week of battles between President Bashar al-Assad's forces and his opponents in Damascus, fighting intensified in Aleppo, a more populous commercial city that long seemed immune to the 16-month-old upheaval convulsing Syria. Rebels seeking to capture downtown Aleppo were combating Syrian troops and intelligence men at the gates of the Old City, a U.N. World Heritage site, residents and activists said. The deaths in the prison mutiny were caused when Assad's forces used machineguns and teargas on inmates overnight, activists in contact with surviving prisoners said. At least nine people were killed in army shelling of al-Herak, a town south of Deraa, the cradle of the revolt against more than four decades of Assad family rule, activists said. Video posted on the Internet showed the shattered bodies of a veiled woman and six children in colorful pajamas, some of them very young. Four lay on one doctor's table. Activist accounts and videos are hard to verify independently due to Syrian restrictions on media access. In Damascus, explosions and gunfire rocked the central district of Barzeh after government forces stormed in overnight, opposition activists said. Tanks prowled the streets of Midan, a neighborhood recaptured by the army from rebels on Friday. Several shells landed in the southern suburb of Hajar al-Aswad, where Assad's forces have been trying to dislodge rebels. Elsewhere residents buried their dead or ventured back to check on homes they had fled to escape the fighting. Outside Damascus and Aleppo, Assad's forces have used artillery and helicopter gunships to keep rebel fighters off balance in the last few days, while avoiding ground incursions, rebel and opposition sources said, saying Deir al-Zor and al-Herak were among towns suffering such long-range bombardments. As the struggle for Syria intensified, Western leaders seized on an admission by Damascus that it has chemical and biological arms and could use them if foreign powers intervened. US President Barack Obama said the world would hold Assad and his entourage accountable "should they make the tragic mistake of using those (chemical) weapons". Israel, which has publicly discussed military action to prevent Syrian chemical weapons or missiles from reaching Assad's Lebanese Shi'ite militant allies Hezbollah, said there was no sign any such diversion had occurred. "At the moment, the entire non-conventional weapons system is under the full control of the regime," a senior Israeli defense official, Amos Gilad, told Israel Radio. Apparently responding to U.S. and Israeli concerns that Syria's non-conventional arsenal could fall into the hands of Hezbollah or other militants, Damascus acknowledged on Monday for the first time that it had chemical and biological arms. Foreign Ministry spokesman Jihad Makdissi said the army would not use chemical weapons to crush rebels but could use them against forces from outside the country. The Global Security website, which collects published intelligence reports and other data, says there are four suspected chemical weapons sites in Syria: north of Damascus, near Homs, in Hama and near the Mediterranean port of Latakia.