UN leader Ban Ki-moon is gravely concerned about reports of an Israeli air strike on Syria but cannot independently verify what happened, a spokesman said Thursday.
On top of a protest to the United Nations, Syria has also complained to the UN Security Council about the raid, diplomats said.
"The secretary general calls on all concerned to prevent tensions or their escalation in the region," said deputy UN spokesman Eduardo del Buey.
Syria, threatening retaliation, says the raid targeted a military research center while other reports have claimed it hit an arms convoy. Israel has not commented.
"The secretary general notes with grave concern reports of Israeli air strikes in Syria," the spokesman while stressing that the United Nations has no details of the incident nor could it "independently verify what has occurred."
Syria made its formal complaint to the UN Disengagement Observer Force, which monitors a ceasefire zone between Syria and Israel in the Golan Heights.
"UNDOF did not observe any planes flying over the area of separation and therefore was not able to confirm the incident. UNDOF also reported bad weather conditions," del Buey said.
The UN mission in south Lebanon, UNIFIL, had also been unable to confirm any Israeli air strike, del Buey said earlier. But the Lebanon force "can certainly confirm that there were a high number of Israeli overflights that UNIFIL recorded on Tuesday."
Ban called on all sides to "strictly abide by international law, in particular in respect of territorial integrity and sovereignty of all countries in the region."
Syria said it has protested to the United Nations over the incident. The UN secretariat, however, has not confirmed the protest.
The UN Security Council is "monitoring" the heightened tensions, however, Masood Khan, Pakistan's UN ambassador and president of the council for January, told reporters.
"Things are developing very rapidly, this incident has taken place. We have received a communication from the permanent representative of Syria," Khan said.
He gave no details of the Syrian letter, but said there was no request for a special meeting of the 15-country council. "They would give due consideration to this development," Khan added.