Hezbollah’s al-Manar TV said they were killed in Quneitra province “during a field reconnaissance mission”.
One of those killed was Jihad Mughniyeh, son of a top military commander killed in 2008, sources said.
Israel said it would not comment, though unnamed sources confirmed an Israeli helicopter strike.
They claimed those targeted were conducting reconnaissance for a Hezbollah attack.
The incident comes days after a warning to Israel by the Hezbollah leader, Hassan Nasrallah, that his forces might retaliate against what he called repeated Israeli strikes inside Syria.
He said his forces had been stockpiling weapons for such a confrontation and that these included long range missiles that could hit every part of Israel.
Hezbollah militants have been supporting President Bashar al-Assad in a four-year Syrian conflict that activists say has left more than 200,000 people dead.
Israel has conducted several air strikes inside Syria since the conflict began, said to be aimed at preventing the transfer of stockpiles of rockets from the Syrian government or Iran to Hezbollah.
Israel fought a 34-day war with Hezbollah in 2006.
Al-Manar TV said the names of the fighters would be released after their families had been informed.
However, media sources said that among those killed, in addition to Mughniyeh, was Mohammed Issa, a Hezbollah field commander.
Hezbollah blamed Israel for killing Imad Mughniyeh in a bombing in Damascus in 2008. Israel denied it.
Mughniyeh was widely believed to be behind a wave of Western hostage-taking in Lebanon during the 1980s.
Most of the Golan Heights, a rocky plateau in south-western Syria, were seized by Israel from Syria in the closing stages of the 1967 Middle East War. About 200 sq miles remained under Syrian control.
The two countries remain technically in a state of war, and UN observers are deployed to monitor a 70km-long (45-mile) demilitarised zone.