Syrian tank (Wikimedia Commons)

The violence erupted after a relatively calm Saturday when two U.N. observers were in that city. At least 17 people throughout the country were killed on Sunday, including six in Homs, according to the opposition Local Coordination Committees of Syria.

Residents have been pleading for the observers to stay in Homs, an anti-government stronghold leading the charge to oust unpopular President Bashar Assad. The city has been bombarded for months with deadly attacks by regime forces, opposition activists told CNN.

"This U.N. observers thing is a big joke," said Douma-based activist Mohammed Saeed. "Shelling stops and tanks are hidden when they visit somewhere, and when they leave, shelling resumes."

The U.N. mission, which was approved on Saturday, is meant to stabilize a ceasefire that took effect 10 days ago, according to Newsday. U.N. chief Ban Ki-Moon has accused Assad of violating the truce.

“The gross violations of the fundamental rights of the Syrian people must stop at once,” Ban said.

An observer team deployed by the Arab League at the start of the year withdrew after making little progress in calming the violence. Western ambassadors have threatened sanctions if Assad doesn’t comply with the ceasefire.

“The mission will fail in its task if the regime continues to violate its commitments and obstructs the work of the mission,” said British diplomat Mark Lyall Grant.

The Syrian regime has continuously downplayed the uprising, refusing to budge from the claim that it is being attacked by a foreign-led conspiracy of criminals and terrorists. The U.N. predicts that 9,000 people have died since the violence began 13 months ago.

Reach Executive Producer Danny Lee here, or follow him here.