Syrian rebels attacked army positions in the northern province of Aleppo on Saturday while Islamist fighters clashed with Kurdish militias on the border with Turkey, residents said.
Insurgents also attacked troops guarding the strategic Tishrin dam, located on the Euphrates river between the provinces of Aleppo and Raqa.
The rebels have surrounded the area, about 10 kilometres (six miles) from the town of Manbij, local resident Abu Mohammed told AFP.
Opposition fighters already control one of the main routes to Raqa and the Tishrin dam would give them a second passage, connecting a wide expanse of territory between the two provinces, both of which border Turkey.
In Aleppo city, the commercial capital where fighting has reached stalemate after five months of deadly urban combat, clashes broke out near an air force intelligence building, the Syrian Observatory for Human Rights said.
Insurgents earlier this week captured Base 46, just west of Aleppo. Nearly 300 soldiers were killed in the sprawling army garrison, according to the rebels, and a large cache of arms and ammunition seized.
The rebels are aiming to also seize Sheikh Suleiman base, also west of the city, that they have encircled for nearly two months, to give them full control of a swathe of northwest Syria from Aleppo to the Turkish border.
In Hasakeh province, northwest Syria, Ras al-Ain saw its fiercest violence since the town near the Turkish border was captured by rebels two weeks ago, a resident told AFP.
"There are so few people, most have left. There is no electricity, no water and no mobile coverage," said Ali, a farmer in his 40s, who fled with his family on Saturday.
"The fighting has been non-stop for five or six days now, but in the last 24 hours it has gotten worse ... The Kurds are bringing reinforcements from Derik and other nearby villages," he said.
Two main Kurdish groups have joined forces in a standoff with hundreds of Islamist rebels, a Syrian Kurdish representative and an activist said on Friday.
Hundreds of fighters loyal to the Kurdish Democratic Union Party (PYD) -- which has close ties to Turkey's rebel Kurdistan Workers' Party (PKK) -- have been locked in fierce battles with fighters of the jihadist Al-Nusra Front and allied Ghuraba al-Sham group in Ras al-Ain.