BEIRUT — Syrian government troops heavily shelled rebellious districts in the resistance stronghold of Homs Tuesday, killing at least 16 people and compounding fears of a new round of bloody urban combat in a country careening toward all-out civil war.

Activists said the intense shelling of Baba Amr in Homs lasted a few hours but did not seem to be the start of a widely expected military offensive aimed at retaking rebel-held neighbourhoods in the central region. Two of the 16 people killed were children, activists said, warning that Homs is already facing a humanitarian catastrophe.

In the northern province of Aleppo, the government said a Syrian businessman was shot dead in front of his home in what appears to be the latest in a series of targeted that suggest armed factions are growing bolder and more co-ordinated in their uprising against President Bashar Assad.

An activist inside Homs said the shelling started after repeated attempts by troops to storm the edges of Baba Amr.

"Government troops have been unable to advance because of stiff resistance from defectors inside," he told The Associated Press on condition of anonymity, fearing government reprisals.

The military sent columns of tanks and other reinforcements toward Homs on Monday, activists said. A flood of military reinforcements has been a prelude to previous offensives by the authoritarian regime, which has tried to use its overwhelming firepower to crush an opposition that has been bolstered by defecting soldiers and hardened by 11 months of street battles.

On Monday, the Red Cross said it was trying to broker a cease-fire among all parties in Syria to allow emergency aid in. On Tuesday, Russia said the United Nations should send a special envoy to Syria to help co-ordinate security issues and the delivery of humanitarian assistance. Russia's Foreign Ministry said on Twitter that it's proposing that the U.N. Security Council ask the U.N. Secretary General to send the envoy.

Despite the humanitarian activity, activists reported heavy shelling of the Baba Amr, Khaldiyeh and Karm el-Zeytoun districts -- all in Homs. It lasted for more than two hours early in the morning, followed by intermittent attacks concentrated on Baba Amr.

Baba Amr on Homs' southwest edge has become the centre of the city's opposition. Hundreds of army defectors are thought to be taking shelter there, clashing with troops in hit-and-run attacks each day.

Residents and activists say a monthslong siege and stepped up attacks on Baba Amr in recent days have left the district without enough food, medicine, electricity and water.

"They bombed all the water tanks on the roofs of buildings, there's no water. Some people have gone without bread for days," said activist and resident Omar Shaker. "If they don't die in the shelling they will die of hunger," he added.

Shaker, who recently fled from Baba Amr centre to the edges, said at one point in the morning the shells were falling at a rate of around 10 per minute. He said he saw thick grey smoke rise from residential areas. Among the dead were two children, he said. More than 200 others were wounded, he added.

Phone lines have been cut with the city, making it difficult to get firsthand accounts from Homs residents.

One amateur video filmed by activists and posted on the Internet showed thick smoke and shells slamming behind a building in Baba Amr. Another showed a shop on the ground floor of a building on fire, the narrator crying: "We are dying. Where are the Arabs?" to the backdrop of gunfire and shells.

Rami Abdul-Rahman, who heads the Observatory, said 16 people died in the shelling, but added there was no indication yet that a major ground assault to take back Baba Amr had begun.

Shaker called on countries attending a planned "Friends of Syria" meeting in Tunisia at the end of this week to find ways of helping the Syrian people.

"People don't care if it's the devil intervening to save us from Bashar, we need the world's help," he said.

State-run news agency SANA said Syrian businessman Mahmoud Ramadan was shot dead Tuesday by gunmen in the northern province of Aleppo.

Gunmen on Sunday staged a guerrilla-style ambush attack in northern Syria that killed a senior state prosecutor and a judge. On Saturday, a member of the Aleppo city council was also killed. The city has been a centre of support for Assad since the uprising began.

Russia's U.N. Ambassador Vitaly Churkin said Monday the world body should help solve humanitarian issues in Syria.

Russia and China have vetoed two Security Council resolutions backing Arab League plans aimed at ending the conflict and condemning Assad's crackdown on protests that killed 5,400 in 2011 alone, according to the U.N. Hundreds more have been killed since, activist groups say. One of the groups puts the toll at more than 7,300.

Russian Foreign Ministry spokesman Alexander Lukashevich also said Tuesday that Moscow will not attend the planned "Friends of Syria" meeting, because its organizers had failed to invite representatives of the Syrian government.

Lukashevich said the meeting wouldn't help a dialogue, saying that the global community should act as friends of the entire Syrian people, and not just one part.

"It looks like an attempt to forge some kind of international coalition like it was with the setting-up of a 'Contact Group' for Libya," Lukashevich said.

Russia has said it will block any U.N. resolution that could pave the way for a replay of what happened in Libya. In that case, Russia abstained from a vote, which cleared the way for months of NATO air force attacks that helped Libyans end Moammar Gadhafi's regime.

Two Iranian navy ships crossed the Suez Canal Tuesday on their way back from the Syrian port of Tartus. The ships had docked in Syria over the weekend on a mission to provide maritime training to Syria's naval forces, according to Iranian media reports.

Bernardino Leon, the EU's representative for the Southern Mediterranean, said Assad's regime missed the opportunity for reforms.

"Syria is definitely not in a transition despite announcements of changes, despite plans for a referendum," Leon told reporters in Jordan.

In Gaza, some 500 Palestinians gathered in a Hamas-authorized demonstration in solidarity with Syrian protesters.

Assad has long hosted and supported leaders of the Islamic Hamas movement, which rules Gaza. But as the body count in Syria continues to rise, Hamas has been trying to distance itself of late from its patron.