At least 16 soldiers were killed in fighting in the northern province of Aleppo, while two others died in attacks elsewhere in the country.
"The clashes happened almost simultaneously at dawn" in several towns in Aleppo province, the Observatory's Rami Abdel Rahman told AFP by telephone.
The Britain-based group added that a number of soldiers defected after an attack on an artillery battalion in the province, taking with them a large amount of weapons.
In Aleppo city, a minibus driver was shot dead by unidentified gunmen, while in Andan, a man and his wife were killed in regime shelling.
Elsewhere in the province, regime troops stormed the Neirab refugee camp, where there were reports of casualties, according to the Observatory.
In the flash point city of Homs, three rebels were killed on Sunday during pre-dawn clashes, as regime forces bombarded several rebel-held districts in the central city, leaving one civilian dead.
In eastern Deir Ezzor city, four people were killed, among them three rebels shot dead by sniper fire. Elsewhere in the province, a girl was killed in shelling.
In northwestern Syria, three civilians were killed in Idlib province, two of them shot at a military checkpoint in Idlib province, while a rebel died in clashes in the Kurdish Mountain region of Latakia province.
In the southern province of Daraa, a civilian was killed in random shooting by regime troops, the Observatory reported.
"This is one of the bloodiest weeks in the conflict," Abdel Rahman said.
According to the Observatory's figures, 94 people were killed in Syria on Monday, 62 on Tuesday, 88 on Wednesday, 168 on Thursday, 116 on Friday and 116 on Saturday.
"It's like we are in a war," Abdel Rahman said. "Sometimes when two countries are at war, not even 20 people are killed a day. But now in Syria it has become normal to have 100 killed each day."
While the Syrian regime on Sunday fended off accusations it shot down a Turkish jet in international airspace, one of its own helicopters was taken out by rebels near the Jordanian border, according to the watchdog.
In a further blow to the army, rebels captured 11 soldiers during an attack on their base in the regime stronghold of Al-Nabak in Damascus province.
"The morale of the military is very low," Abdel Rahman said, adding the army recently buried 113 soldiers in one region alone.
More than 15,000 people, mostly civilians, have been killed in Syria since the outbreak of the revolt against Assad's rule in March 2011, according to the Observatory.
