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 neighborhoods in the central region. At least two of the 16 people killed were children, activists said, warning that Homs is already facing a humanitarian catastrophe.
The attacks compounded fears of a new round of bloody urban combat in a country careening toward all-out civil war.
"The current situation requires an immediate decision to implement a humanitarian pause in the fighting," said Jakob Kellenberger, the president of the Geneva-based International Committee of the Red Cross.
"In Homs and in other affected areas, entire families have been stuck for days in their homes, unable to step outside to get bread, other food or water, or to obtain medical care," he said in a statement.
The Red Cross said it has been negotiating with Syrian authorities and members of the opposition to agree a temporary cease-fire so that emergency aid can reach beleaguered parts of the country.
"It should last at least two hours every day, so that ICRC staff and Syrian Arab Red Crescent volunteers have enough time to deliver aid and evacuate the wounded and the sick," said Kellenberger.
Also Tuesday, Russia said the United Nations should send a special envoy to Syria to help coordinate 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.
The U.N. estimates that at least 5,400 people, most of them civilians, have been killed in the 11-month uprising against Syria's President Bashar Assad. But that figure was given in January and hundreds more have been reported killed since.
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 coordinated.
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 months of street battles.
Activists reported heavy shelling of the Baba Amr, Khaldiyeh and Karm el-Zeytoun districts 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 center 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 center 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 gray 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.
A main opposition group, the Local Coordination Committees, said 30 people were killed in Homs and 20 others elsewhere in the country, including 17 in Idlib. The figures could not immediately be confirmed by others.
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 center of support for Assad since the uprising began.
Also Tuesday, four United Nations human rights investigators urged Syria to free a group of at least 16 activists, bloggers and journalists who were arrested by authorities last week, saying they risked being tortured in detention.
The U.N. experts said those detained Feb. 16 in a raid on the Syrian Center for Media and Freedom of Expression in Damascus included activist Mazen Darwish and U.S.-born blogger Razan Ghazzawi. They cited reports that the activists were blindfolded and taken to a detention center run by Syria's military intelligence on the outskirts of Damascus.
Russia and China have vetoed two Security Council resolutions backing Arab League plans aimed at ending the conflict and condemning Assad's crackdown. Moscow also has promised to block any future U.N. resolution, fearing a repeat of the mandate for international action in Libya.
Russian Foreign Ministry spokesman Alexander Lukashevich 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.
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Jordans reported from Geneva. Associated Press writer Jamal Halaby contributed to this report from Amman, Jordan.
