(AFP) - Hezbollah chief Hassan Nasrallah warned Friday that war between Israel and Hamas could turn into a regional conflict if attacks on Gaza continue, placing responsibility firmly on the United States.

In his first speech since war broke out almost four weeks ago between Hamas militants and Israel, the head of the powerful Iran-backed movement warned that "all options" were open for an expansion of the conflict to Lebanon.

"America is entirely responsible for the ongoing war on Gaza and its people, and Israel is simply a tool of execution," Nasrallah said in a televised broadcast, calling the conflict "decisive".

"Whoever wants to prevent a regional war -- and this is addressed to the Americans -- must quickly stop the aggression on Gaza," he said.

Thousands of supporters gathered to hear the fiery speech at an event in Beirut's southern suburbs, a Hezbollah stronghold, held in memory of the group's fighters killed in Israeli bombardments.

Others gathered elsewhere in Lebanon and the region, including Tehran and Baghdad.

Since Hamas militants launched a shock October 7 attack on Israel from the Gaza Strip, Lebanon's southern border has seen escalating tit-for-tat exchanges, mainly between Israel and Hezbollah, an ally of the Palestinian group, stoking fears of a broader conflagration.

US President Joe Biden has sent two aircraft carrier groups to the eastern Mediterranean.

But a defiant Nasrallah told the United States that "your fleet in the Mediterranean does not scare us... we are ready to face the fleet you threaten us with."

- 'Unimaginable price' -

"You Americans know well that if there is war in the region, your fleet will be of no use, nor will air combat help. Your interests and your soldiers and your fleet will be the first to pay the price," he added.

In Washington, White House Press Secretary Karine Jean-Pierre said Hezbollah "should not try to take advantage of the ongoing conflict".

"This has the potential of becoming a bloodier war between Israel and Lebanon than 2006," she said. "The United States does not want to see this conflict expand into Lebanon."

Israeli Prime Minister Benjamin Netanyahu warned Hezbollah it would "pay an unimaginable price" for any misstep.

The 2006 conflict between Israel and Hezbollah killed more than 1,200 people in Lebanon, mostly civilians, and 160 in Israel, largely soldiers.

Israeli officials say Hamas's bloody raids on October 7 killed more than 1,400 people, most of them civilians.

The health ministry in Hamas-run Gaza says more than 9,200 people have died in Israeli bombardments, mostly women and children.

Nasrallah saluted "heroic" Hamas, calling the October 7 attacks "100 percent Palestinian" and saying the plan was not disclosed to allies in advance.

He praised attacks targeting US bases in the Middle East, after a string of assaults on facilities hosting US troops in Iraq and Syria.

Hezbollah along with armed groups from Iraq, Syria and Yemen are part of a regional, Iran-led "axis of resistance" against Israel and the United States.

- 'Foolishness' -

Nasrallah also warned Israel against attacking Lebanon, saying that "all options are open on our Lebanese front".

The current situation at the Lebanon-Israel border is linked "to the course and development of events in Gaza", Nasrallah said, claiming Hezbollah's actions had tied up "a large section" of the Israeli army that might otherwise have been fighting in the Palestinian territory.

He warned that the chance of open conflict was "realistic".

"We say to the enemy that might think of attacking Lebanon or carrying out a pre-emptive operation, that this would be the greatest foolishness of its existence," he said.

Cross-border skirmishes have killed 72 people on the Lebanese side, among them at least 54 Hezbollah fighters but also other combatants and civilians, one a Reuters journalist, according to an AFP tally.

On the Israeli side, at least six soldiers and one civilian have been killed, the army said.

Maha Yahya of the Carnegie Middle East Center, said Nasrallah's speech was "the best middle ground that he could take".

Ultimately "neither Iran nor Hezbollah are interested in getting into a conflict which would probably end up being a zero sum game", she said.

© Agence France-Presse