UN Secretary-General Ban Ki-moon on Sunday, May 5 voiced alarm at reports Israel has struck targets inside Syria, but said the United Nations was unable to confirm whether any such attacks had taken place, Reuters reported.

"The Secretary-General expresses grave concern over reports of air strikes in Syria by the Israeli Air Force," Ban's press office said in a statement.

"At this time, the United Nations does not have details of the reported incidents," it said. "Nor is the United Nations in a position to independently verify what has occurred."

It added that Ban "calls on all sides to exercise maximum calm and restraint, and to act with a sense of responsibility to prevent an escalation of what is already a devastating and highly dangerous conflict."