The army of President Bashar Assad has been on the offensive in Homs province in recent weeks, reclaiming some of the territory it has lost to the rebels since Syria's crisis began 27 months ago.
The military, building on its capture of the strategic town of Qusair between the Lebanese border and Homs at the beginning of this month, has overrun a number of nearby villages. It also has hammered the center of the city, a rebel stronghold since the uprising against Assad began in March 2011.
Homs, a city of about 1 million, has shown great sympathy for the opposition since the early days of the uprising. A month after it started, protesters carried mattresses, food and water to the main Clock Square, hoping to emulate Cairo's Tahrir Square, the epicenter of Egypt's revolt that overthrew Hosni Mubarak.
Security forces quickly raided the encampment, shooting at protesters and chasing them through the streets. The onslaught only boosted the intensity of the protests, fueling a revolt that has posed the most serious challenge to date to the Assad family dynasty that has ruled Syria since 1970.
Homs is the capital of Syria's largest province, which carries the same name and stretches from the Lebanese border to the frontier with Jordan and Iraq.
Activists in the city said all cellular lines were cut early Saturday before warplanes pounded rebel-held areas. The air raids were followed by intense shelling with artillery, mortars and tanks, before troops tried to advance.
Several activists in the city said the regime began bringing in reinforcements since last week, apparently in preparation for the attack.
Two activists said about 400 shells struck rebel-held areas such as Qusour, Jouret el-Shayah, Old Homs and Khaldiyeh.
"This is the worst campaign against the city since the revolution began," said an activist in the rebel-held old quarter of the city via Skype. "They are using all types of weapons," said the man on condition of anonymity for fear of government reprisals.
The Britain-based Syrian Observatory for Human Rights said air strikes hit two districts in the center of the city. It said the army also fired mortar shells into the neighborhoods.
An activist from the neighborhood of Khaldiyeh said tanks were also involved in the bombardment, and that the military was trying to push into the area from all sides.
Shelling has been continuous since 10 a.m. in that area and in nearby Old Homs, activist Tariq Bardakhan told The Associated Press via Skype.
"Today is one of the most violent days that Homs has witnessed since the beginning of the revolution," he said.
In an activists' video of the bombardment, several large explosions can be heard as plumes of grey smoke rise from buildings in a densely built-up area of the city.
The narrator of the video says: "These are heavy explosions that hit Homs, God is great." Another shell lands and smoke can be seen rising from behind a mosque. Two minarets are seen in the distance and the narrator says they belong to the historic Khalid Ibn al-Walid mosque in Khaldiyeh.
The video was posted on the Internet on Saturday and appears consistent with AP's reporting from the area.
The Observatory confirmed clashes around the mosque, and said that part of the building, which dates back to the 13th century and has been damaged in previous fighting, was engulfed in flames. It added that troops tried to storm the mosque with no success.
The Observatory said both sides have sustained casualties, but did not have numbers.
Syrian state TV said the army has had "great success" in the battle for Homs after "killing many terrorists in the Khaldiyeh district."
Syrian state media refers to rebels fighting to oust Assad from power as "terrorists" and say they are mercenaries of the West and their Gulf Arab allies who are conspiring against Damascus.
Before the fighting moved to the capital Damascus and the northern city of Aleppo in July last year, Homs was the center of the uprising and became known as "the capital of the Syrian revolution."
Rebels received a major blow in March last year when troops captured the Baba Amr neighborhood after weeks of fighting that left scores dead. Among those killed in Baba Amr last year were French photographer Remi Ochlik and Britain's Sunday Times correspondent Marie Colvin.
After the army captured the neighborhood, Assad paid a visit to the area in a show of how important Homs is for the government. The city lies along a land corridor linking two of Assad's strongholds, the capital of Damascus and an area along the Mediterranean coast that is the heartland of his minority Alawite sect.
The Observatory says more than 100,000 people have been killed in the Syrian conflict since it began as peaceful protests against the Assad regime more than two years ago. It became an armed rebellion after the opposition supporters took up arms to fight a government crackdown.
The United Nations puts the number of casualties at 93,000.
Also Saturday, the Observatory and the Aleppo Media Center said a missile hit Aleppo's Katourji neighborhood, killing and wounding several people. The Observatory said at least three people were killed while the AMC said the death toll could be as high as 15.
An amateur video showed two buildings that had several top stories knocked out. Panicked residents ran to help evacuate wounded people, including children. A boy, his head covered with a bloodied white cloth was being rushed away as people chanted "God is great."
Another man carried a wounded child and ran in a street filled with debris. At least one dead person was seen carried away.
The video appeared genuine and corresponded to other AP reporting on the events depicted.
The military has gained momentum after capturing Qusair earlier this month with the help of fighters from Lebanon's Hezbollah group, capturing villages on the roads linking the capital to the border area with Jordan and Lebanon.
The rebels have also claimed some victories, marking a successful end to a two-week battle in the south Friday by capturing an army checkpoint in the city of Daraa, the provincial capital of the region that carries the same name.
Daraa is the birthplace of the uprising against Assad and rebels hope to one day launch an offensive from there to take the capital.
The Observatory reported heavy fighting around the province on Saturday with clashes between the rebels and army troops concentrated in the town of Jassem after the army brought in reinforcements.
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