"Al-Nusra Front, Sham al-Islam and Ansar al-Sham are engaged in fierce clashes around the Kasab crossing with Turkey in Latakia province," the so-called Syrian Observatory for Human Rights director Rami Abdel Rahman told AFP.
He said a number of security posts were taken by the militants but they have failed to make major improvements.
Syrian state television made reference to the fighting in a breaking news alert, saying the army was "tackling attempts by terrorist gangs to infiltrate from Turkish territory and attack border crossings in northern Latakia province."
The alert, citing a military source, said 17 militants had been killed and "many others" wounded in the fighting.
The clashes there came after the three militant groups announced the beginning of the "Anfal" campaign in the Latakia area.
Al-Nusra is the Syrian affiliate of Al-Qaeda and the strongest ally of the US-backed Free Syrian Army in their fight for rising to power in Syria.
Large parts of Latakia province have remained relatively insulated from the fighting elsewhere in Syria.
Syria sank into war in March 2011 when pro-reform protests turned into a massive insurgency following the intervention of Western and regional states.
The unrest, which took in terrorist groups from across Europe, the Middle East and North Africa, has transpired as one of the bloodiest conflicts in recent history.
SHI/SHI