The attacks, claimed by the Islamic State of Iraq and the Levant (ISIL) and Al-Nusra Front, two Al-Qaeda-affiliated groups, came after rebels were driven out of nearby Qara village yesterday, said the Syrian Observatory for Human Rights.
"At least seven regime troops were killed in the two attacks, and five others are in critical condition," Observatory director Rami Abdel Rahman said.
Regime loyalists backed by Lebanon's Hezbollah movement have launched a major assault on Qalamoun, a mountainous region straddling key supply lines between Damascus and Homs, as well as rebel smuggling routes criss-crossing the border with Lebanon.
A Syrian security source told AFP there was just one explosion at a checkpoint at the entrance to Nabek.
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"The soldiers at the checkpoint stopped a suspicious car and the driver, who was a suicide bomber wearing an explosive belt, tried to escape but was shot dead by soldiers," the source said.
"However, the vehicle exploded," the source added, saying there had been "victims".
Fighting raged elsewhere in Qalamoun, mainly around the rebel bastion of Yabrud, which came under army shelling today, and Deir Attiya, a regime stronghold where clashes with rebels erupted for the first time.
The army had earlier said it was "hunting" jihadists across the region, while rebels vowed to return to Qara.
Elsewhere, a mortar round landed near central Damascus' Umayyad square, while two others hit the Kassaa neighbourhood of the capital, said the Observatory.
Some 120,000 people are estimated to have been killed in Syria's uprising, which began with peaceful protests against President Bashar al-Assad's regime in March 2011 but escalated into a full-blown insurgency when his troops launched a brutal crackdown.