Syrian activists say violence has killed at least 23 people as more U.N. observers began deploying to back a cease-fire plan that has failed to end more than a year of unrest.

The Britain-based Syrian Observatory for Human Rights says rebels killed 12 soldiers on Tuesday, in a battle in the northeastern province of Deir Ezzor. It says one civilian was killed when government troops responded with rockets and machine guns. Elsewhere, the Observatory says a mortar shell struck a village in the northwestern province of Idlib, killing 10 people, nine of them from one family.

Casualties could not be independently confirmed.

The small team of U.N. monitors in Syria has grown to 30 personnel, with more expected to arrive in the coming weeks. The U.N. Security Council has authorized a 300-strong observer mission, but it is not clear when it will be fully deployed.

In a statement on Tuesday, U.N. Secretary-General Ban Ki-moon repeated his call for all sides in Syria's 13-month conflict to stop armed violence and cooperate with the U.N. observers to implement the April 12 cease-fire agreement. He also condemned a recent series of bombings in the town of Idlib and in the capital, Damascus, calling them “terrorist” attacks.

Syrian state media say a double suicide bombing in Idlib killed at least nine people on Monday, while suicide car bombing in Damascus killed at least nine others on Friday. An Islamist group calling itself the al-Nusra Front claimed responsibility for the Damascus attack.

The Syrian government blames the bombings on “armed terrorists,” a term it uses for rebels leading an uprising against autocratic President Bashar al-Assad. But activists accuse the government of orchestrating the attacks to discredit the opposition.

The United Nations estimates that at least 9,000 people have been killed since Mr. Assad began cracking down on the uprising in March 2011.