Syrian government forces killed 21 people and wounded some 340 on Tuesday when they unleashed a heavy artillery barrage on a rebel-held district (Baba Amr) of the city of Homs, central Syria, one day after the army sent reinforcement ahead of a possible ground assault, activists said. Activists said the heavy shelling of the Baba Amr, Khaldiyeh and Karm el-Zeytoun lasted more than two hours.

The bombardment rained down as International Committee of the Red Cross officials tried to negotiate a pause in the fighting to allow them to bring aid to civilians suffering in horrendous conditions after 18 days of attacks on Homs. In Damascus, security forces opened fire on demonstrators overnight, wounding at least four, activists said, in the latest sign that the 11-month-old uprising against President Bashar al-Assad is taking a grip on the capital as well as the provinces.

Western powers and the Arab League prepared for a meeting of the "Friends of Syria" contact group in Tunisia on Friday to pressure Assad to step down, while Russia and China backed Assad's own program for reforms.

Russia said it would not attend the meeting because the Syrian government would not be represented. The Russian Foreign Ministry suggested the United Nations Security Council should send a special humanitarian envoy to Syria.

Activists said government forces launched the artillery attack on Homs after rebel fighters holding the opposition Baba Amro district blocked troops from entering.