Russia is expressing concern about an airstrike by Israeli warplanes inside Syria.
The Russian Foreign Ministry says such action, if confirmed, amounts to "unprovoked attacks" against a sovereign nation, in violation of the United Nations charter.
Differing accounts of the Wednesday airstrike have emerged, with Syrian authorities saying Israeli jets fired on a military research facility near Damascus, killing two people.
Israeli and Western news media reported an Israeli airstrike at a different location close to the Syrian-Lebanese border. They say the target was a convoy delivering missile parts to Hezbollah, the strongly anti-Israel Shi'ite militia based in Lebanon.
The conflicting reports could not be resolved or independently confirmed by early Thursday and it remains unclear whether one or two separate strikes occurred. Israel routinely declines to acknowledge preemptive military actions and refused all comment Wednesday.
In recent weeks, Israeli officials have warned that they will not tolerate any transfer of Syrian weapons to militants such as Hezbollah.
The former head of the Israeli intelligence agency Mossad, Amnon Sofin, says Israel's greatest concern is that Syrian chemical weapons could come under control of Hezbollah militants dug in along the Lebanese border.
Sofin told reporters Wednesday that Hezbollah already has rockets and launchers and there are fears that such missiles could be fitted to carry chemical warheads.
A statement from Syria's military command describes the early-morning Israeli attack as "a direct strike on a scientific research center." It says the strike followed months of "botched attempts" to seize control of the facility by "terrorist groups" - the government's label for rebels trying to topple President Bashar al-Assad. The Israeli pilots are said to have flown into Syria at low altitude, to evade detection.