Israel warned Russia it would "know what to do" if the delivery went ahead, and Syria's top rebel commander gave Hezbollah, the powerful Lebanese Shiite movement, a 24-hour ultimatum to stop fighting alongside regime forces.
The developments stoked tensions after the European Union decided to lift an embargo on weapons to Syria's rebels, in a move the opposition reacted to with caution.
Syria's regime joined its ally Russia in condemning the EU decision as an "obstruction" to peace efforts, while accusing the bloc of supporting and encouraging "terrorists".
"We consider these supplies a stabilising factor," deputy foreign minister Sergei Ryabkov said, adding they could act as a deterrence against foreign intervention.
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Israel has strongly objected to the delivery, and its defence minister warned of a response.
"The deliveries have not taken place, and I hope they do not. But if, by misfortune, they arrive in Syria, we will know what to do," said Moshe Yaalon.
Israel has reportedly carried out at least three strikes against Syria since the uprising against President Bashar al-Assad erupted in March 2011, apparently targeting weapons sites.
The army said the attack came in the early hours near Arsal, a northeastern Lebanese town in where most residents back Syria's uprising.
And in the eastern Lebanese town of Hermel, a Hezbollah stronghold, security sources said six rockets apparently fired from Syria landed in the area throughout the day, wounding seven people.
Hezbollah is allied with the Syrian regime and fighting alongside the army against rebels, including in the central town of Qusayr, where it has lost dozens of men.
"If the attacks of Hezbollah against Syrian territory do not stop within 24 hours, we will take all measures to hunt Hezbollah, even in hell," he told Al-Arabiya news channel.
The tensions overshadowed an ongoing meeting in Istanbul of the National Coalition, Syria's main opposition group, which responded cautiously to the EU's decision to lift its arms embargo on the rebels.
"Definitely it is a positive step, but we are afraid it could be too little, too late," Coalition spokesman Louay Safi told AFP.