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Some of the dead were shot after they had surrendered, according to video footage. Rebels berated them, calling them "Assad's Dogs", before firing round after round into their bodies as they lay on the ground.
The highway linking the capital Damascus to the contested city of Aleppo, Syria's commercial centre, has been the scene of heavy fighting since rebels cut the road last month. Saraqeb lies about 40km south of Aleppo
In other developments, China put forward a new initiative to resolve the 19-month-old conflict, including a phased, region-by-region ceasefire and the setting up of a transitional governing body.
A Chinese Foreign Ministry spokesman said Beijing had made the proposal to international peace envoy Lakhdar Brahimi - whose own call for a truce over the Muslim holiday of Eid was largely ignored by both sides.
The United States meanwhile has called for an overhaul of Syria's opposition leadership, signalling a break with the largely foreign-based Syrian National Council to bring in more credible figures.
A meeting in Qatar next week of foreign powers backing the rebels will be an opportunity to broaden the coalition against President Bashar al-Assad, U.S. Secretary of State Hillary Clinton said in Zagreb on Wednesday.
The United States and its allies have struggled for months to craft a credible opposition coalition, while Assad has counted on the support of Russia, Iran and, to a lesser extent, China. International efforts to end the violence have all foundered.
More than 32,000 people have been killed since protests against Assad, an Alawite who succeeded his late father Hafez in ruling the mostly Sunni Muslim country, first broke out on city streets. The revolt has since degenerated into full-scale civil war, with the goverment forces relying heavily on artillery and air strikes to thwart the rebels.
The army has lost swathes of land in Idlib and Aleppo provinces but is fighting to control towns along supply routes to Aleppo city, where its forces are fighting in many districts.
The head of the pro-opposition Syrian Observatory for Human Rights, Rami Abdelrahman, said two of the attacked checkpoints at Saraqeb were on the Damascus-Aleppo highway. The third was near a road linking Aleppo with Latakia, a port city still mostly controlled Assad's forces.
"The rebels will not stay at the checkpoints for long as Syrian warplanes normally bomb positions after rebels move in," Abdelrahman said.
Five rebels died in the fighting and at least 20 soldiers were killed at the third site, including those shot after surrendering, he said.
The video footage showed a group of petrified men, some bleeding, lying on the ground as rebels walked around, kicking and stamping on their captives.
One of the captured men says: "I swear I didn't shoot anyone" to which a rebel responds: "Shut up you animal ... Gather them for me." Then the men are shot dead.
Reuters could not independently verify the footage.
The Observatory said the al Qaeda-inspired Jabhat al-Nusra rebel group was responsible for the executions.
Islamist rebel units are growing in prominence in the war - a cause for concern for international powers as they weigh up what kind of support to give the opposition.
U.S. President Barack Obama's administration has said it is not providing arms to internal opponents of Assad and is limiting its aid to non-lethal humanitarian assistance.It concedes, however, that some of its allies are providing lethal assistance.
Russia and China have blocked three U.N. Security Council resolutions aimed at increasing pressure on the Assad government, leading the United States and its allies to say they could move beyond U.N. structures for their next steps.
China has been strongly criticised by some Arab countries for failing to take a stronger stance on the conflict. Beijing has urged the Assad government to talk to the opposition and take steps to meet demands for political change.
"More and more countries have come to realise that a military option offers no way out, and a political settlement has become an increasingly shared aspiration," Chinese Foreign Ministry spokesman Hong Lei said in Beijing.
He said China's new proposal was aimed at building international consensus and supporting peace envoy Brahimi's mediation efforts.