An Israeli air strike has killed six members of Hezbollah in the Syrian sector of the Golan Heights, the Lebanese militant movement says.
Among those reported dead were the son of a late military leader, a current commander, and at least one Iranian.
Hezbollah's al-Manar TV said they were killed in Quneitra province "during a field reconnaissance mission".
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.
Those who died include Jihad Mughniyeh, the son of a top military commander killed in 2008, and Mohammed Issa, a Hezbollah field commander, Hezbollah officials said.
One member of Iran's Revolutionary Guards had died, the Britain-based Syrian Observatory for Human Rights said. Iran's semi-official Tabnak news agency said several Revolutionary Guards had been killed.
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.
Following Sunday's strike, al-Manar said Israel was "playing with fire that puts the security of the whole Middle East on edge".
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, a mainly Shia group backed by Iran, in 2006.
Jihad Mughniyeh is the son of Imad Mugniyeh, who was killed in a bombing in a bombing in Damascus in 2008. Hezbollah blamed Israel for the death, but Israel denied it.
Imad 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.
Who are Hezbollah?
- Name means "Party of God"
- Political and military organisation made up mainly of Shia Muslims
- One of the biggest blocs in Lebanon's governing coalition
- Strongly backed by Iran, a close ally of Syrian President Assad
- Designated a terrorist organisation by Western states, Israel and Gulf Arab countries