Israel's military said sirens sounded in the Israel's north on Thursday after missiles were fired from Iran, on the seventh day of war between the two countries.

A military statement said that "sirens sounded in several areas across Israel following the identification of missiles launched from Iran toward the State of Israel", with the army's Home Front Command saying air raid warnings were activated in large parts of the north.

The Israeli military said Thursday that Iran used a missile with multiple warheads in its attack, posing a new challenge to its defenses.

Instead of having to track one warhead, missiles with multiple warheads can pose a more difficult challenge for air defense systems, like Israel’s Iron Dome.

For years, Israel has targeted Iranian nuclear scientists, hoping to choke progress on Iran's nuclear program by striking at the brains behind it.

Now, with Iran and Israel in an open-ended direct conflict, scientists in Israel have found themselves in the crosshairs after an Iranian missile struck a premier research institute known for its work in life sciences and physics, among other fields.

Much of the focus on Iran's nuclear program has been on Tehran's enrichment of uranium, but experts also keep a close watch on the Islamic Republic's Arak heavy water reactor.

That's because the facility, some 250 kilometers (155 miles) southwest of Tehran, could produce plutonium, which can be used to make an atomic bomb.

Harakat al-Nujaba, one of the main Iran-backed militant groups in Iraq, threatened to attack U.S. citizens in the event that Iran’s Supreme Leader Ayatollah Ali Khamenei is targeted in the ongoing Israel-Iran conflict.

Iraqi militias have thus far largely held their fire in the conflict, although three drones launched at the Ain al-Asad base housing U.S. troops in western Iraq were reportedly shot down on Friday, after Israel’s began its barrage of strikes on Iran. No group claimed responsibility for the attack on the base.

Iran's deputy foreign minister warned the United States on Thursday against intervening in the war to back up its ally Israel, adding that his country was ready to defend itself in case of escalation.

"If the United States wants to actively enter the field in favor of the Zionist regime, Iran will have to use its tools to both teach a lesson to aggressors and defend its national security and national interests," said deputy foreign minister Kazem Gharibabadi, according to state TV.

Gaza's civil defense agency said that Israeli forces on Thursday killed at least 18 people, including 15 who had gathered near an aid distribution site in central Gaza.

Civil defence official Mohammad al-Mugghayyir told AFP that "18 people have been killed due to ongoing Israeli shelling on the Gaza Strip since dawn today, 15 of whom were waiting for aid", adding that the remaining three were killed by shelling near Gaza City.

Israel's defense minister said Thursday that Iran's supreme leader Ayatollah Ali Khamenei "can no longer be allowed to exist" after an Israeli hospital was hit during an Iranian missile attack.

"Khamenei openly declares that he wants Israel destroyed -- he personally gives the order to fire on hospitals. He considers the destruction of the state of Israel to be a goal," Israel Katz told journalists in Holon near Tel Aviv. "Such a man can no longer be allowed to exist."

Russian President Vladimir Putin and his Chinese counterpart Xi Jinping on Thursday strongly condemned Israeli attacks on Iran in a phone call, the Kremlin said, adding that both leaders called for a diplomatic solution to the conflict.

"Both sides adhere to identical approaches, strongly condemn Israel's actions," Kremlin aide Yuri Ushakov told reporters, adding that Moscow and Beijing believed the end to the hostilities "should be achieved exclusively by political and diplomatic means".

Iran’s foreign minister will meet with European counterparts in Geneva as an Israeli airstrike campaign continues to target his country, state media reported Thursday.

Foreign Minister Abbas Araghchi will travel to Geneva for the meetings Friday, the state-run IRNA news agency report.