Iran has so far played no direct role in the conflict which has spread through the region since the onset of the Gaza war six months ago, whilst backing groups which have taken part by attacking Israel, U.S. interests, and Red Sea shipping.
Built up over years and in some cases decades of Iranian support, the groups describe themselves as the "Axis of Resistance" to Israel and U.S. influence in the Middle East.
The Axis includes Hamas, the Palestinian group which ignited the war by attacking Israel on Oct. 7.
LEBANON'S HEZBOLLAH
Hezbollah, meaning "Party of God", was set up by Iran's Revolutionary Guards in 1982 with the aim of fighting Israeli forces that had invaded Lebanon that year. Hezbollah is widely regarded as more powerful than the Lebanese state and shares Iran's Shi'ite Islamist ideology.
Hezbollah has served as a model for other Tehran-backed groups across the region, some of which it has advised or trained. The United States and other governments including U.S. allied Gulf Arab countries have listed Hezbollah as a terrorist organisation.
The heavily armed Lebanese group has been mounting near daily attacks on Israeli targets at the Lebanese-Israeli border since early October, prompting the heaviest exchanges of fire between the enemies since they waged full-scale war in 2006.
Hezbollah says its attacks have helped to stretch the Israeli army while also uprooting tens of thousands of Israelis who have fled homes near the border. Israeli air and artillery strikes have also forced tens of thousands of Lebanese to flee.
Israeli strikes have killed some 240 Hezbollah fighters including top commanders in Lebanon since Oct. 7, in addition to a further 30 killed in Israeli strikes in Syria, according to security sources.
Overall, this equals Hezbollah's losses in the 2006 war.
A U.S. envoy has been engaged in efforts to prevent the violence from spiralling into an even bigger conflict.
YEMEN'S HOUTHIS
The Houthi movement established control over large parts of Yemen during a civil war that began in 2014 when they seized Sanaa and overthrew the government. They belong to the Zaydi sect of Shi'ite Islam and have long had friendly ties to Iran.
The Yemen war pitched them into conflict with Saudi Arabia and its Gulf allies who, wary of Iran's broadening sway, intervened in the war in 2015 in support of the ousted government. Saudi Arabia has in recent years backed diplomatic efforts to end the war, last September hosting Houthi negotiators in Riyadh.
The Houthis announced on Oct. 31 they had entered the conflict by firing drones and missiles towards Israel.
In November, they expanded their role by attacking shipping in the southern Red Sea, saying they were targeting vessels belonging to Israelis or heading to Israeli ports - though some of the targeted ships had no known Israeli links.
The campaign prompted the United States and Britain to launch air strikes against Houthi targets in Yemen in January. The Houthis declared all U.S. and British ships and warships taking part in "the aggression" would be targets for the group.
The attacks have disrupted international commerce on the shortest shipping route between Europe and Asia, leading some shipping companies to reroute their vessels.
The United States believes Iran's Revolutionary Guards (IRGC) have been helping to plan and carry out the Houthi missile and drone attacks. Iran denies involvement.
Sources told Reuters in January that commanders from the IRGC and Hezbollah are on the ground in Yemen helping to direct and oversee Houthi attacks on Red Sea shipping. The Houthis denied any Hezbollah or Iranian involvement.
The United States returned the Houthis to its list of designated terrorist groups in response to the shipping attacks.
The Saudi-led coalition fighting the Houthis has long accused Iran of arming, training and funding the group. The Houthis deny being an Iranian proxy.
THE ISLAMIC RESISTANCE IN IRAQ
Shi'ite groups with ties to Iran emerged as powerful players in Iraq after the 2003 U.S.-led invasion, and developed militias with tens of thousands of fighters.
An umbrella group of Shi'ite Islamist armed factions called the Islamic Resistance in Iraq began targeting U.S. forces stationed in Iraq and Syria in October, saying they aimed to respond to Israeli attacks on Palestinians and to resist U.S. forces deployed in Iraq and the region.
The attacks stopped after a drone strike killed three U.S. soldiers in Jordan on Jan. 28, prompting heavy retaliatory U.S. airstrikes on Iran-linked targets in Syria and Iraq.
Reuters reported in February that the head of Iran's Quds Force asked the militias to lie low to avoid U.S. strikes on their senior commanders, destruction of key infrastructure or even a direct retaliation against Iran.
On April 1, The Islamic Resistance in Iraq claimed responsibility for an aerial attack on Eilat in Israel.
Shi'ite armed groups played a leading role in battling the ultra-hardline Sunni Islamist Islamic State group, fighting as part of Hashd al-Shaabi, or the Popular Mobilisation Forces.
While members of these armed groups receive state salaries and are technically under the authority of the prime minister, they often operate outside the chain of command.
The groups which targeted U.S. forces included Kataib Hezbollah and the Nujaba group, both of which are closely linked to Iran's Revolutionary Guards. Their arsenal includes explosive drones, rockets and ballistic missiles.
The groups are designated terrorist organisations by the United States.
SYRIA
The Syrian government led by President Bashar al-Assad is part of the Axis of Resistance but has not played any direct role in the current conflict.
However, Syrian territory has been an arena of escalation.
This has included attacks by Iran-backed militias on U.S. forces in the east, Israeli airstrikes in Syria against Iranian personnel and members of Hezbollah, and occasional exchanges of fire between the Israeli-occupied Golan Heights and southwest Syria.
The Syrian government has for decades been a close ally of Iran, and Iran-backed forces have deployed across much of Syria since arriving more than a decade ago to aid Assad in the Syrian civil war. Tehran and Damascus says Iranian forces are in Syria in an advisory role at the government's invitation.
