Israel announced the launch of "precise strikes" on military targets in Iran on Saturday in retaliation for Iranian attacks, as an AFP journalist in Tehran reported hearing several explosions.
Israel had vowed to hit back at Iran for its October 1 missile strike, the second-ever direct attack by the Islamic republic on its arch-foe.
The rapid escalation of violence has heightened fears across the region and beyond of a wider war, pitting Israel against Iran and the allies it dubs the "axis of resistance" on multiple fronts.
Since the deadliest attack in its history on 7 October, 2023, Israel has been fighting Hamas in Gaza, and since late last month, it has been at war with Hezbollah in Lebanon.
Both Hezbollah and Hamas are allies of Iran, as are armed groups in Yemen and Iraq and the government in Syria.
In a statement, the Israeli military said that it was conducting "precise strikes on military targets in Iran", in response to what it said were "months of continuous attacks from the regime in Iran".
"The regime in Iran and its proxies in the region have been relentlessly attacking Israel since October 7th -- on seven fronts -- including direct attacks from Iranian soil," the military said.
Iran in April launched its first-ever direct assault on Israeli territory in retaliation for a deadly strike on Iran's consular annex in Damascus.
Defence Minister Yoav Gallant had said that Israel's retaliation for the October 1 strike would be "deadly, precise and surprising".
The October 7, 2023, attack on Israel that sparked the conflict killed 1,206 people, mostly civilians, according to an AFP tally of official Israeli figures.
Israel's retaliatory campaign in Gaza has killed 42,847 people, the majority civilians, according to figures from the Hamas-run territory's health ministry which the United Nations considers reliable.
It has since broadened the scope of its operations to Lebanon, vowing to secure its northern border after nearly a year of attacks launched by Iran-backed Hezbollah in support of Hamas.
At least 1,580 people have been killed in Lebanon since September 23, according to an AFP tally of Lebanese health ministry figures.
"Our defensive and offensive capabilities are fully mobilized," the Israeli military said.
Military spokesman Daniel Hagari in a separate statement urged people to be "alert and vigilant".
Iran's October 1 strike came after an Israeli air raid killed Hezbollah chief Hassan Nasrallah and Revolutionary Guards general Abbas Nilforoushan in Lebanon on September 27.
The killing of Nasrallah, which dealt Hezbollah a seismic blow, followed the death of Hamas's political leader, Ismail Haniyeh, on July 31 in Iran in an attack widely blamed on Israel.
Iranian state television reported blasts around the capital Saturday, saying they were due to the "activation of the air defense system" against an Israeli attack.
"The loud blasts heard around Tehran were related to the activation of the air defense system against the actions of the Zionist regime (Israel) which attacked three locations outside of Tehran city," state TV reported, citing officials of the Tehran province air defense.
An AFP reporter also heard the explosions.
Iranian media said no fires or blasts had been reported at a main oil refinery near the capital.
Iran had in recent weeks warned that any attack on its "infrastructure" would provoke an "even stronger response", while Revolutionary Guards general Rassul Sanairad said an attack on nuclear or energy sites would cross a red line.
US National Security Council spokesman Sean Savett said the "targeted strikes on military targets" are "an exercise of self-defense and in response to Iran's ballistic missile attack against Israel on October 1."
The United States was "informed beforehand and there is no US involvement," a US defense official told AFP, on condition of anonymity.
The official did not say how far in advance the United States had been informed or what had been shared by Israel.
The scope and nature of Saturday's strike was not immediately clear.
But Syrian state news agency SANA said Israel conducted an air attack on Saturday from the occupied Golan Heights and Lebanon, targeting military positions in Syria.
Citing an unnamed military source, the agency said the attack at around 2:00 am (2300 GMT) had targeted positions in central and southern Syria.
The attack prompted Syria to activate its air defenses, SANA said, as Israel announced it was launching "precision strikes" in neighboring Iran.
Iran and Syria are allies in the so-called "axis of resistance" that also includes Hezbollah in Lebanon and Hamas in Gaza.
"Our anti-aircraft defense is confronting hostile targets in the skies around Damascus," state news agency SANA reported on Telegram.
SANA had previously reported the "sounds of explosions" in the vicinity of the Syrian capital.
The Islamic Resistance in Iraq, a loose network of pro-Iran factions, claimed responsibility early Saturday for a drone attack against a "military target" in northern Israel, following the announcement of Israeli strikes on Iran.
Iraq suspended all air traffic Saturday at its airports until further notice after Israel its attack.
by Jay DESHMUKH