The UN chief has called "for an immediate and unconditional de-escalation of violence" after Israel launched intensive air raids on what it called Iranian bases in Syria, raising tensions in the region.

"[A]ll concerned in Syria and the region have a responsibility and must abide by international law and relevant Security Council resolutions," Antonio Guterres said in a statement on Saturday.

"The secretary-general is following closely the alarming military escalation throughout Syria and the dangerous spillover across its borders," the UN chief's spokesman Stephane Dujarric said.

The Israeli army on Saturday launched "large-scale" attacks inside Syria after intercepting what it said was an Iranian drone that entered occupied Golan Heights and the subsequent downing of an Israeli fighter jet by Syrian regime air defense forces.

The UN chief's reaction came after Russia, which intervened in support of Bashar al-Assad in 2015, said they were "seriously concerned" by the confrontation.

US support

The White House expressed its support for Israel after the hostilities.

"Israel is a staunch ally of the United States, and we support its right to defend itself from the Iranian-backed Syrian and militia forces in southern Syria," the White House statement released late on Saturday read.

"We call on Iran and its allies to cease provocative actions and work toward regional peace."

The Pentagon and the Department of State made similar comments earlier on Saturday.

The Syrian regime military and its allies denied that the drone violated Israel's airspace, Syria's state-run SANA news agency reported.

Sources in Damascus said there were casualties among Syrian regime forces.

The Syrian Observatory for Human Rights, a UK-based monitor, said Israel had targeted several bases in the east of the central province of Homs.

It said the bases are used by both Iranians and Russians deployed in support of the regime.

Israel is technically at war with Syria and occupies a swath of the strategic Golan Heights that it seized in the Six Day War of 1967 and later annexed in a move never recognized by the international community.

Israel has sought to avoid direct involvement in the Syria conflict but acknowledges carrying out dozens of air raids there to stop what it calls deliveries of advanced weaponry to Hezbollah.

Al-Jazeera