Russian diplomats refrain from confirming that Assad sent letter to US via Moscow

"We are so far receiving information from open sources the same way you are. Therefore, we cannot give any special commentary on behalf of the embassy at the moment. We are following ourselves what is being published," said an embassy spokesperson.

The Kuwaiti newspaper Alrai reported earlier that Assad had sent a letter to Washington through Moscow to warn that Israel's new aggression would evoke an immediate response. It said Assad expected an answer from Washington within 24 hours.

The newspaper cited a source close to Assad as saying that the Syrian leader told Russia he expected an answer to his letter sent to the U.S. within 24 hours and warning that, if Israel attacks Syria again, it would interpret this as a declaration of war and would retaliate without prior notice.

Syrian authorities reported earlier that Israel had delivered an air strike on a research center outside Damascus in the early hours of Sunday.

Israel has neither confirmed nor denied the attack. Israeli media suggested that the target of the air strike was not the research center but Iranian missiles shipped through Syria to Hezbollah in Lebanon.

"At least 15 soldiers were killed, and dozens more are missing" after the strikes near Damascus early on Sunday, said the Observatory's Rami Abdel Rahman.Israeli strikes on Syrian military targets at the weekend killed at least 15 soldiers and dozens more were unaccounted for, the Syrian Observatory for Human Rights watchdog said on Monday.

"These three sites (targeted) would usually have around 150 soldiers in them, but it's not clear if they were all there at the time of the strikes."

Syria's government said the explosions were the second Israeli airstrike in three days. The latest target, officials said, was a military research facility outside the Syrian capital. A top Syrian official told CNN in an exclusive interview that the attack was a "declaration of war" by Israel.A series of massive explosions illuminated the dark sky over Damascus early Sunday, igniting renewed claims that Israel has launched attacks into the war-torn country.

Syrian authorities vowed to retaliate against Israel but did not specify what action they would take.

The Israeli military would not confirm or deny the Syrian claim that Israel fired rockets that hit the Jamraya research center in the Damascus suburbs.

"We do not comment on these reports at all," an Israeli military representative said.