The attacks all took place in the city of Idlib, activists and state media reported. The city, which is in Syrian government hands, is the local capital of Idlib province.
Today's attacks were the most serious there since Syrian rebels took control of scores of villages and towns around it more than two years ago.
The fighting is separate from the clashes underway between Nusra Front's main rebel rivals, the Islamic State group, and Syrian Kurdish fighters for control of the strategic border town of Kobani, further to the east and along the border with Turkey.
"It was a moral blow to the regime," said activist Asad Kanjo, based in the town of Saraqeb, also in Idlib province. He added that calm had been restored in the city.
Syria's pro-government Al-Ikhbariya TV cited the provincial police chief, who was not named, as saying the attackers took advantage of a power cut before dawn to hit the checkpoints and also the governor's office. He added that troops repelled the attackers.
The Observatory said the rebels were helped by some policemen who were protecting the police commend and the governor's office enter the two buildings. The buildings were retaken later by government troops.
Another activist in Idlib province, speaking on condition of anonymity for fear of reprisals, said most of the attacks took place on the southern edge of the city, near Mastoumeh Hill.
The Observatory said the hill was captured by rebels, which prompted Syrian helicopter gunships to target the site. It said 15 rebels and 20 soldiers were killed at the hill.
State TV said the attackers were led by Abu Waleed al-Libi who was killed in the battle al-Libi is Arabic for "the Libyan." The Observatory said that some of the Nusra Front members killed in the fighting were foreigners.
In the eastern province of Deir el-Zour a car bomb exploded near a hospital run by the Islamic State group killing four people including a child and wounding others, the Observatory said. The explosion in the town of Mayadeen caused material damage to the hospital and nearby homes.
The state channel and the Observatory gave no further details.
The Islamic State group controls wide areas of the oil-rich Deir el-Zour province, where some tribesmen rose against the extremist group but were quickly crushed.
In Kobani, an Associated Press journalist on the Turkish side of the border said there was intense fighting in the town today. Sporadic explosions and occasional cracks of gunfire could be heard from a distance.
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