An Israeli strike on Thursday hit an arms supply hub operated by the Lebanese Shia Hezbollah group near Damascus airport where regular supplies of weapons from Tehran are sent by commercial and military cargo planes, a regional intelligence source said.
The depot handles a significant amount of weapons that Iran, a major regional ally of Syrian regime leader Bashar al Assad, sends regularly by air, the source said.
The source said the arms depot gets a major part of the weapons supplied to an array of Iranian-backed militias, led by Hezbollah, which have thousands of fighters engaged in some of the toughest fronts against regime opposition groups.
Russia criticised the Israeli raid and warned Tel Aviv "to avoid any action that heightens tension in the region."
Kremlin spokesman Dmitry Peskov said Syria's sovereignty should be respected, adding the Russian and Israeli militaries were in constant contact.
A spokeswoman for the Israeli military, asked if Israel had been involved in carrying out air strikes targeting Damascus airport, said, "We can't comment on such reports."
Two senior opposition sources operating in the Damascus area cited their monitors in the eastern outskirts of the capital, where the airport is located, as saying that five strikes had hit an ammunition depot used by Iran-backed militias.
Hezbollah-linked Al-Manar television said initial indications were that the blast had caused only material damage and no human casualties.
Assad is backed in the war by Russia, Iran and regional Shia militias including Hezbollah, which is a close ally of Tehran and a sworn enemy of Israel.
Israeli officials have cited any movement of advanced weaponry to Hezbollah units in Syria as a "red line" that has prompted it to carry out air strikes or artillery fire in the past.