Syria's nearly week-old cease-fire continued to deteriorate Wednesday as a bombing killed six Syrian law enforcement officers and government forces renewed shelling in the flashpoint Homs region.

The state-run SANA news agency says an “armed terrorist group” detonated an explosive device planted on a road in the northern Idlib province. It also says a sniper killed a police officer in the southern city of Daraa.

Activists say Syrian forces are continuing to pound rebel-held neighborhoods in the flashpoint city of Homs

The Syrian government has said it has the right to respond to attacks from what it calls “terrorists,” despite its agreement to a peace plan brokered by U.N.-Arab League envoy Kofi Annan. The plan also calls for opposition groups to cease violence.

The Britain-based Syrian Observatory for Human Rights, an opposition group that aggregates casualty reports from activists inside the country, said pro-Assad forces killed at least 23 civilians Tuesday as attacks appeared to be expanding to other areas.

The attacks come as a U.N. mission is attempting to monitor cease-fire conditions.

Moroccan Col. Ahmed Himiche, head of an advance team of six U.N. observers that arrived in Syria this week, said the group's first field trip came Tuesday to Daraa.

United Nations chief Ban Ki-moon is intensifying efforts to get a large contingent of observers on the ground to salvage the truce. He said a team of 250 monitors, as originally envisioned, may not be sufficient.

He has also asked the European Union for planes and helicopters to make the mission more effective.

The U.N. says the crackdown in Syria has killed more than 9,000 people since March 2011.

In another development on Wednesday, news organizations said Turkey intercepted a ship suspected of carrying weapons and ammunition to Syria, a move that could breach an arms embargo on Damascus.

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