Violence spread to Syria's largest city, Aleppo, on Friday with two blasts outside security compounds that left 28 people dead.

The explosions outside military intelligence and police compounds were blamed on terrorists by the state media. Some 175 people were injured, the worst day Aleppo has seen since the uprising against President Bashar al-Assad began last year. The northern city and economic hub has been largely quiet, but protests had been planned for Friday. Anti-Assad activists accused the regime of setting off the blasts to discredit the opposition and disrupt demonstrations.

One of two bomb blasts sites in Syria's northern city of
Aleppo. Photograph: Sana/Reuters

In Homs, government forces continued their siege of rebel-held districts and other opposition areas, going house to house arresting people in the Insha'at district and keeping up an artillery and tank barrage on Baba Amr.

The intensified campaign began with the failure of the U.N. Security Council to agree on a common position last weekend, when Russia and China vetoed a resolution backing an Arab League peace plan and calling on Assad to step down. Moscow and Beijing stuck to their positions on Friday, dashing any residual hopes of a diplomatic breakthrough in the security council. Russia's deputy foreign minister, Sergey Ryabkov, accused the west of arming the rebel Free Syrian Army.

"Western states inciting Syrian opposition to uncompromising actions, as well as those sending arms to them, giving them advice and direction, are participating in the process of fomenting the crisis," he said, according to Itar-Tass news agency.