- NEW: At least 30 of the 120 people killed on Saturday died in Aleppo, LCC said
- Heavy clashes engulf Aleppo and at least 30 people have died there
- The French president is calling for U.N. Security Council intervention
- One opposition group says the death toll in the fighting has now surpassed 20,000
Editor's note: Are you in Syria? Share your stories, videos and photos with the world on CNN iReport.
Northern Syria (CNN) -- Tanks pounded Aleppo neighborhoods on Saturday, a sign that a much-feared government offensive in Syria's largest city has started.
Government forces shelled Aleppo neighborhoods, and Free Syrian Army rebels squared off with government soldiers in and around the city on Saturday, the opposition Local Coordination Committees of Syria said.
Syrian TV said "terrorists" suffered heavy losses after clashes in three neighborhoods.
"The heaviest clashes since the beginning of the revolution are taking place now in a number of neighborhoods in Aleppo city," the LCC said.
For more than a week, regime forces and rebel fighters have clashed in Aleppo, Syria's commercial hub and largest city, and the regime has fired artillery from warplanes.
Fighters have been preparing for a major confrontation, and security forces continued military buildup with reinforcements from Raqqa province and other regions, opposition groups said.
Deama, an activist in Aleppo, told CNN that the expected massive military operations have begun since it is the first day the regime is moving into the city with tank fire.
CNN isn't using her full name because disclosing it could put her in danger.
"Many people have been killed," she said, including a mother and her two children. "More tanks have arrived at a military school in northern Aleppo and started shelling for the first time from this location today."
She said the regime is shelling displaced people from Homs and Idlib who've taken refuge in Aleppo, and thousands of them have had to evacuate yet again.
The uprising started in March 2011 after the Bashar al-Assad government cracked down fiercely on peaceful protesters.
More than 20,000 people, mostly civilians, have died in the conflict, the LCC said. This is one of several overall death toll estimates from various opposition groups and from the United Nations. U.N. Secretary-General Ban Ki-moon, for example, said at one point this week that almost 17,000 people have died.
Libyan rebels move onto Syrian battlefield
At least 30 of the 120 people that the LCC reported killed across the country on Saturday died in Aleppo province, it said.
Securing Aleppo is key for the embattled regime and the anti-government fighters. One Free Syrian Army commander said the regime has labeled the confrontation for the city as the "mother of all battles."
Regime forces are preventing fuel and food from entering Aleppo neighborhoods controlled by rebel fighters, opposition activists said. Rebels have had to set up medical clinics in homes.
U.S. increasing contacts with Syrian rebels
"They are besieging our area," said Abu Omar, a resident of the Salaheddin neighborhood. "There is no electricity in some parts, and food is scarce."
Deama said the humanitarian situation is "disastrous."
"We have a bread crisis because the regime apparently ordered all bread bakeries closed. But in areas under the control of the Free Syrian Army, the FSA opened the bakeries and were able to employ people to bake bread then distribute it. There are also severe shortages in medications," she said.
The United Nations and Western countries, fearing widespread death and destruction, have urged the Syrian government to call off an offensive in Aleppo. French President Francois Hollande reiterated his position calling for the U.N. Security Council to urgently intervene in Syria, a source in his entourage confirmed. Speaking in southwestern France, Hollande didn't specify what kind of intervention is needed.
Heavy violence on Saturday wasn't limited to Aleppo. The LCC said at least 22 people died in violence across Damascus and its suburbs, and others died in Idlib, Deir Ezzor, and Hama provinces. Syrian state-run media alluded to a possible showdown in Hama province.
"In a response to the constant pleas of the residents in the area, a unit from our armed forces started to cleanse Karnaz town in Hama from the armed terrorist gangs," Syrian state TV reported. The Syrian government has blamed violence in the country on vaguely defined armed terrorist gangs.
Syrian rebels appear more capable, yet still outgunned
Fighting over the Syrian issue flared across the border in the Lebanese city of Tripoli on Saturday.
At least 12 people were injured in clashes between Alawites and Sunnis. In Syria, Alawites, an offshoot of Shiism, dominate al-Assad's regime. While Sunnis have a significant presence in the Syrian regime, they are also dominant in the opposition movement.
Friends of al-Assad's regime signaled their support for the embattled regime.
Iran's energy minister, Majid Namjou, vowed his government "will not leave Syria alone in such a difficult situation," according to state-run Press TV.
The report said the two nations signed a deal Thursday to expand sharing of electricity and water, with Namjou saying Iranian firms are ready to rebuild damaged power plants.
A.K. Lukashevich, the Russian foreign ministry spokesman, on Saturday reiterated his country's opposition to unilateral sanctions on Syria and won't agree to inspections of ships sailing under the Russia flag.
Russia has been a longtime arms supplier to Syria, but it has said it will not deliver new weapons to Syria as long as the situation there is unstable. Lukashevich made the remarks after the latest round of European Union sanctions against the al-Assad regime.
The civil war is making an impact on foreign entities based in Syria.
Poland has closed its embassy in Damascus and evacuated its diplomats because of the bad security situation, the Foreign Ministry said on Friday.
Navy Chief Vice Adm. Viktor Chirkov said on Saturday that personnel at the Russian naval base in the Syrian port of Tartus may be evacuated if it comes under attack, state-run RIA Novosti reported. He said the base is needed for maintenance and technical support to Russian warships in the Mediterranean Sea and for crafts on anti-piracy missions.
Syria has faced high-level diplomatic and military defections in recent months.
The latest is Farouq Taha, the Syrian ambassador to Belarus. He confirmed the move on Al-Jazeera TV on Saturday.
The Obama administration supports political transition and has called for al-Assad to step aside. One senior U.S. official told CNN the United States has increased contacts with the Syrian opposition in recent weeks.
"The U.S. and others are playing more of an advisory role to the opposition now," the official said Friday.
But U.S. Joint Chiefs Chairman Gen. Martin Dempsey said Friday he's not aware of any discussions about giving the opposition any "lethal support."
U.S. keeping an eye on Syria's weapons, al Qaeda's presence
CNN's Saad Abedine, Amir Ahmed, Schams Elwazar, Joe Sterling, Ivan Watson and Barbara Starr and journalist Shiyar Sayed Mohamad contributed to this report.
