Iran warned on Saturday it would defend itself after Israeli air strikes killed at least two soldiers and further stoked fears of a full-scale war in the Middle East.
Israel warned Iran would "pay a heavy price" if it responded to the strikes, and the United States, Germany and Britain demanded Tehran not escalate the conflict further.
The European Union called for all parties to exercise utmost restraint to avoid an "uncontrollable escalation" in the Middle East, warning: "The dangerous cycle of attacks and retaliations risks causing a further expansion of the regional conflict."
Other countries, including many of Iran's neighbours, condemned Israel's strikes and some, such as Russia, urged both sides to show restraint and avoid what Moscow dubbed a "catastrophic scenario".
The Islamic republic insisted it had the "right and the duty" to defend itself, while its Lebanese ally Hezbollah said it had already launched rocket salvos targeting five residential areas in northern Israel.
Confirming its own strikes after explosions and anti-aircraft fire echoed around Tehran, the Israel military said it had hit Iranian missile factories and military facilities in several regions.
The "retaliatory strike has been completed and the mission was fulfilled", while Israeli aircraft "returned safely", a military spokesman said.
Iran confirmed Israel had targeted military sites around the capital and in other parts of the country, saying the raids caused "limited damage" but killed four soldiers.
Israel had vowed to retaliate after October 1, when Iran fired around 200 missiles in only the second ever direct attack against its arch-foe. Most of those missiles were intercepted but one person was killed.
The Israeli retaliation drew condemnation from Iraq, Pakistan, Syria and Saudi Arabia, which warned against further escalation. Jordan said Israeli jets had not used its airspace. Turkey was one of the most outspoken critics, calling for an end to "terror created by Israel".
Israel is already engaged in combat on two fronts.
Since September, Israel has stepped up its aggression in Lebanon, carrying out daily strikes in the country's south, Beirut and other regions. Israel subsequently invaded the country in early October. The attacks have killed 2,653 as of Friday, with around 1,580 killed since September 23.
Israel has been carrying out a war in Gaza for over a year that has killed at least 42,924 Palestinians in the densely populated territory.
Israel's atrocities in the Gaza Strip have been labelled as a genocide by experts, UN agencies and NGOs.
The United Nations has warned the "darkest moment" of that conflict is unfolding, with Palestinians facing a dire humanitarian crisis and daily Israeli bombing.
Along with Hezbollah and Hamas, Iranian-allied groups in Yemen, Iraq and Syria, have carried out attacks during the fallout from the Gaza war.
At roughly the same time as Israel struck targets in Iran, the Syrian state news agency SANA said an Israeli air attack targeted military positions in central and southern Syria.
The Islamic Resistance in Iraq, a loose network of pro-Iran factions, claimed responsibility before dawn Saturday for a drone attack against a "military target" in northern Israel.
Hezbollah said it had also fired rockets at Israeli soldiers near the south Lebanon village of Aita al-Shaab and launched drones against an Israeli air base south of Tel Aviv.
On Saturday, Lebanon's health ministry said an Israeli strike had killed a Hezbollah-affiliated medic in Bazuriyeh in the south of the country.
US National Security Council spokesman Sean Savett said Israel's response to Iran was "an exercise in self-defence".
He urged Iran to "cease its attacks on Israel so that this cycle of fighting can end without further escalation".
Israel, however, was the first to attack Iran. In April, an Israeli strike on Iran's consular annex in the Syrian capital Damascus killed commanders of its Islamic Revolutionary Guard Corps.
Israel and Iran have since been engaged in infrequent, tit-for-tat attacks, with the last being Tehran's launching of 200 missiles on October 1 which they said was in retaliation for the assassinations of Hezbollah leader Hassan Nasrallah and Hamas' politburo chief Ismail Haniyeh, who was killed in Iran.
Prior to Israel's wars in Gaza and Lebanon, Iran and Israel have been engaged in a proxy war for years.