An international rights group Wednesday accused Syria of committing war crimes, while activists said rebels killed 15 security force members in what could be another sign a U.N.-brokered cease-fire has frayed.

Human Rights Watch says Syrian forces and pro-government militias killed at least 95 civilians between late March and early April in the northwestern Idlib province, in the lead-up to the April cease-fire brokered by international envoy Kofi Annan.

In a Wednesday report, the group's Anna Neistat said it was as if government forces “used every minute before the ceasefire to cause harm.”

The Syrian Observatory for Human Rights says rebels killed 15 security force members in an ambush in Aleppo province on Wednesday.

The the incident came a day after rights groups and activists said at least 30 people were killed in anti-government related violence, including nine family members an Idlib province village.

U.N. observers say the Syrian military still has heavy weapons stationed in cities, and that the government and opposition have both violated a peace plan aimed at ending the unrest.

Meanwhile, mission spokesman Neeraj Singh says the number of military observers in Syria rose to 31 on Wednesday.

“The teams are already in the area, as you know, Homs, Hama, Daraa, idlib, so in their areas they will be carrying out activities.”

Under the U.N. plan, the number of monitors in Syria will eventually increase to 300.

The United Nations estimates at least 9,000 people have been killed since President Bashar al-Assad began cracking down on an uprising against him in March of last year.