Explainer-What is Hezbollah, the group backing Hamas against Israel?

BEIRUT (Reuters) -Hostilities between Lebanon's Hezbollah and Israel have rumbled on in parallel to the Gaza war for more than six months, fuelling concern about the risk of a wider conflict between the heavily armed adversaries.

Underlining that risk, Israel said on Sunday that it had completed another step in preparing for a possible war along its northern front.

This is a primer on Hezbollah and its role in the war:

WHAT ARE HEZBOLLAH'S ORIGINS?

Iran's Revolutionary Guards founded Hezbollah in 1982 during Lebanon's 1975-90 civil war, part of Iran's effort to export its 1979 Islamic Revolution and fight Israeli forces which had invaded Lebanon in 1982.

The group has risen from a shadowy faction to a heavily armed force with big sway in Lebanon and the region. Western governments including the United States designate it a terrorist group. So do Sunni Muslim Gulf Arab states including Saudi Arabia.

Hezbollah is a Shi'ite Islamist group and shares the ideology of the Islamic Republic of Iran.

HOW DID HEZBOLLAH GET INVOLVED IN THIS WAR?

Hezbollah is a powerful part of the "Axis of Resistance", an alliance of Iran-backed groups across the Middle East that also includes the Palestinian Islamist movement Hamas, which ignited the Gaza war by attacking Israel on Oct. 7.

Declaring solidarity with the Palestinians, Hezbollah began firing on Israeli positions in the frontier region on Oct. 8.

The sides have been trading fire on a near daily basis since then, with Hezbollah launching rockets and drones and Israel mounting air and artillery strikes. The attacks have mostly struck near or at the frontier, but both sides have also widened their attacks in recent weeks.

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. Overall, this equals Hezbollah's losses in the 2006 war with Israel.

As of April 3, at least 66 civilians have been killed in Lebanon, according to a U.N. report. Attacks from Lebanon have killed 18 people - soldiers and civilians - on the Israeli side.

Tens of thousands have been uprooted in Lebanon and Israel.

HOW POWERFUL IS HEZBOLLAH'S MILITARY?

While other groups disarmed after Lebanon's civil war, Hezbollah kept its weapons to fight Israeli forces that were occupying the predominantly Shi'ite Muslim south of the country. Years of guerrilla warfare led Israel to withdraw in 2000.

Hezbollah demonstrated military advances in 2006 during a five-week war with Israel, which erupted after it crossed into Israel, kidnapping two soldiers and killing others.

Hezbollah fired thousands of rockets into Israel during the conflict, in which 1,200 people were killed in Lebanon, mostly civilians, and 158 Israelis were killed, most of them soldiers.

Hezbollah's military power grew after 2006. The group says its rockets can strike all parts of Israel and its arsenal includes precision missiles.

During the Gaza war, Hezbollah has announced attacks using surface-to-air missiles - a weapon it was long believed to have in its arsenal but had never before confirmed possessing. It has also launched explosive drones at Israel.

Hezbollah leader Sayyed Hassan Nasrallah has said the group has 100,000 fighters. The U.S. Central Intelligence Agency's World Factbook says Hezbollah was estimated in 2022 to have 45,000 fighters, split between roughly 20,000 full-time and 25,000 reservists.

WHAT REGIONAL SWAY DOES HEZBOLLAH HAVE?

Hezbollah has inspired and supported other Iranian-backed groups across the region, including Iraqi Shi'ite militias. It played a big part in helping its ally President Bashar al-Assad fight the war in Syria, where it still has fighters.

Saudi Arabia says Hezbollah has also fought in support of the Iran-allied Houthis in Yemen. Hezbollah denies this.

WHAT IS HEZBOLLAH'S ROLE IN LEBANON?

Hezbollah's influence is underpinned by both its weaponry and the support of many Lebanese Shi'ites who say the group defends Lebanon from Israel. It has ministers in government and lawmakers in parliament.

Lebanese parties opposed to Hezbollah say the group has undermined the state and unilaterally dragged Lebanon into wars.

It entered Lebanese politics in 1992, contesting elections, and began taking a more prominent role in state affairs in 2005 after Syria withdrew forces from Lebanon following the killing of former prime minister Rafik al-Hariri, a Sunni politician who symbolised Saudi influence in Beirut.

A U.N.-backed court convicted three Hezbollah members in absentia over the assassination. Hezbollah denies any role, describing the court as a tool of its enemies.

In 2008, a power struggle between Hezbollah and its Lebanese political foes led to armed conflict, after the government vowed to take action against the group's military communications network. Hezbollah fighters took over parts of Beirut.

In 2018 Hezbollah and allies who support its possession of arms won a parliamentary majority. This was lost in 2022, but the group still has major political sway.

ACCUSED OF ATTACKS ON WESTERN INTERESTS

Lebanese officials and Western intelligence have said groups linked to Hezbollah carried out suicide attacks on Western embassies and targets, and kidnapped Westerners in the 1980s.

The United States holds Hezbollah responsible for suicide bombings in 1983 that destroyed the U.S. Marine headquarters in Beirut, killing 241 servicemen, and a French barracks, killing 58 French paratroopers. It also says Hezbollah was behind a suicide attack on the U.S. embassy in Beirut in 1983.

Referring to those attacks and hostage-taking, Hezbollah leader Hassan Nasrallah said in a 2022 interview that they were carried out by small groups not linked to Hezbollah.

Hezbollah has also been accused of militant attacks elsewhere. Argentina blames it and Iran for the deadly bombing of a Jewish community center in Buenos Aires in which 85 people died in 1994 and for an attack on the Israeli embassy in Buenos Aires in 1992 that killed 29 people.

Both deny any responsibility.

(Writing by Tom Perry; Editing by Edmund Blair, William Maclean)