Syrian security forces heavily shelled rebel-held areas in the flashpoint city of Homs Tuesday, killing at least 16 people and wounding more than 200 as fears mounted of an all-out assault on the battered protest hub.

Activists said several shells per minute were falling on the Sunni Muslim neighborhood of Baba Amr, where at least two children died amid the barrage. They said government forces launched the attack after rebel fighters holding the opposition district blocked troops from entering.

The casualty figures could not be independently verified because phone lines to the city have been cut and Syria restricts the operations of foreign media.

The International Committee of the Red Cross Tuesday called on Syrian authorities and rebels to immediately implement a daily two-hour cease-fire so it can deliver emergency aid and evacuate the sick and wounded.

ICRC President Jakob Kellenberger said that in Homs and elsewhere, entire families have been isolated for days, unable to secure food, water or medical care.

The renewed assault on Homs comes as the Britain-based Syrian Observatory for Human Rights said a military convoy of more than 50 vehicles, including tanks and personnel carriers, was seen heading from the capital, Damascus, toward the besieged central city.

The Syrian army has already deployed tanks and other reinforcements around Homs in preparation for a possible ground assault after bombarding it for more than two weeks.

The city is a major hub of the 11-month uprising against the autocratic rule of Syrian President Bashar al-Assad. Syria's military is under the control of Shi'ite Alawite officers, from the same minority sect as Mr. Assad, raising concerns the country is headed toward open civil war.

Activists said security forces also opened fire on demonstrators overnight in Damascus and that troops, police and pro-government militia in Hama have set up dozens of roadblocks and isolated whole neighborhoods.

The escalation comes as Russia, a key ally of Mr. Assad, said it will not attend an international conference on Syria later this week because the Syrian government would not be represented. Russia's foreign ministry said the United Nations Security Council should send a special humanitarian envoy to Syria.

The “Friends of Syria” contact group – comprising Western and Arab nations opposed to Mr. Assad's crackdown – are planning to use Friday's meeting in Tunisia to increase pressure on the Syrian government to halt the bloodshed. Russia and China back Mr. Assad's own reform program, which the Syrian opposition has soundly rejected.

Earlier this month, Moscow and Beijing vetoed a Western- and Arab-backed effort to pass a Security Council resolution endorsing an Arab League plan for Mr. Assad to cede power. But Arab League Secretary-General Nabil Elaraby said Monday he believes Russia and China may be shifting their positions on the Syrian crisis.

Human rights activists say the violence has killed at least 6,000 people.