Syrian rebels have unleashed an unprecedented barrage of mortar fire against government troops in Aleppo after announcing a “decisive” battle for Syria’s second city, residents and activists say.

Shells crashed down at a steady rate and clashes were widespread on Friday, leaving layers of dust and smoke over Aleppo, according to the residents and the Syrian Observatory for Human Rights (SOHR), the UK-based opposition watchdog.

“The fighting is unprecedented and has not stopped since Thursday. The clashes used to be limited to one or two blocks of a district, but now the fighting is on several fronts,” the SOHR’s Rami Abdel Rahman told AFP news agency.

Residents of Aleppo neighbourhoods previously spared the worst of the two-month-old battle for the city told AFP the violence was “unprecedented”.

“The sound from the fighting … has been non-stop,” said a resident of the central district of Sulimaniyeh, who identified himself only as Ziad. “Everyone is terrified. I have never heard anything like this before.”

Rebels claimed they had advanced on several fronts, particularly in the southwest, but admitted they had failed to make any significant breakthrough.

“On the Salaheddin front, we took one of the regular army bases,” said Abu Furat, one of the leaders of the Al-Tawhid Brigade, the most important in the city.

But he admitted that the fighters had to retreat from Salaheddin because they were outgunned. “To win a guerrilla street war, you have to have bombs and we don’t,” he said.

Abu Furat said that 25 soldiers were killed in the assault, while another rebel fighter said 20 of his comrades died on the battlefield and 60 were wounded.

The accounts of violence could not be independently verified by Al Jazeera as Syria restricts access for journalists.

Civilian deaths claimed

The SOHR which gave initial estimates of 60 people killed across the country on Friday – half of them civilians – said at least five civilians and five rebels died in Aleppo.

By Friday afternoon the intensity of the fighting abated, as rebels appeared to focus their attention on other objectives, such as Omayyad Mosque in the centre of the Old City, an AFP correspondent said.

The SOHR’s Abdel Rahman said the fighting was not yielding major gains for either side. “Neither the regime nor the rebels are able to gain a decisive advantage,” he said.

The outgunned rebels, a force made up of mutinous soldiers and civilians who have taken up arms to oust President Bashar al-Assad’s regime, declared an all-out assault for Aleppo on Thursday.

Afterwards, mortars were fired about every 15 minutes into army-held areas, including Sulimaniyeh and Sayyid Ali.

Just north of Aleppo, a Syrian shell crashed into a town on the Turkish side of the border, wounding a Turkish national, as fighting raged in a nearby Syrian town, a local official said.

The shell fired from the border town of Tall al-Abyad landed in Akcakale in the province of Sanliurfa, smashing into the walls of two buildings and slightly wounding one person.

Violence also raged in Damascus where troops attacked several rebel areas in both the north and the south of the capital, leaving three civilians dead, the SOHR said.

Despite the violence, thousands of protesters took to the streets of Aleppo and other cities in support of the unification of the Free Syrian Army as factionalisation appears to undermine the anti-regime revolt.

The SOHR said demonstrations were held after the main weekly Muslim prayers in the Fardus and Sukari neighbourhoods of Aleppo, as well as in the central province of Homs, Hama further north and Idlib in the northwest.

US announcements

The developments came as Leon Panetta, US defence secretary, said that the Syrian regime had moved some of its chemical weapons to safeguard the material.

Panetta, citing US intelligence, said in Washington DC on Friday that he believed that the main storage sites for Syria’s arsenal remained secure.

It was not clear when the movement took place, or even if it was recent, but Panetta said it had occurred in more than one case.