Countries around the world have condemned a wave of attacks by the Palestinian militant group Hamas on Israel that started on Saturday (October 7) and left hundreds dead and thousands wounded.

Many accused Hamas of breaking into homes and "massacring civilians."

Over 600 Israelis and 300 Palestinians were killed in the first day of fighting, Israeli and Palestinian sources report.

The Israeli army said more than 1,000 people in Israel were wounded by gunshots or the more than 3,000 incoming rockets.

Gun battles raged into Sunday morning between Israeli forces and hundreds of Hamas fighters in many locations, including at least two where gunmen were holding hostages, the army said.

Israeli news website Ynet said there were "dozens of Israeli captives, including numerous women, children and elders."

US President Joe Biden pledged "rock solid" support for Israel in the face of what he described as "terrorist assaults" by Hamas.

High-level discussions between US and Israeli officials on military aid were already under way, a US senior administration official said.

Biden described the assault as "a terrible tragedy on a human level" and said he had spoken to Israeli Prime Minister Benjamin Netanyahu to underline his support.

"I told him the United States stands with the people of Israel in the face of these terrorist assaults," Biden said in a televised address from the White House.

"In my administration, support for Israel's security is rock solid and unwavering," he said.

"We'll make sure that they have the help their citizens need and they can continue to defend themselves."

Defense Secretary Lloyd Austin reaffirmed Washington's commitment.

"Our commitment to Israel's right to defend itself remains unwavering, and I extend my condolences to the families of those who lost their lives in this abhorrent attack on civilians," Austin said in a statement. "Over the coming days the Department of Defense will work to ensure that Israel has what it needs to defend itself and protect civilians from indiscriminate violence and terrorism."

As the attacks threatened to trigger a wider conflict, Biden also warned "this is not a moment for any party hostile to Israel to exploit these attacks to seek advantage. The world is watching."

Lebanon's Iran-backed Hezbollah said Sunday it fired artillery shells and guided missiles at Israeli positions in a contested border area.

Biden stressed that Israel -- which the United States has supplied with billions of dollars of arms -- has "a right to defend itself".

US Secretary of State Antony Blinken reiterated that right.

"The United States unequivocally condemns the appalling attacks by Hamas terrorists against Israel," he said. "We will remain in close contact with our Israeli partners. The United States supports Israel's right to defend itself."

Blinken also spoke with Palestinian president Mahmoud Abbas -- whose West Bank-based Fatah movement is a rival to Hamas -- and "called on all leadership in the region to condemn" the attack on Israel.

"There is never any justification for terrorism," said White House National Security Council spokesperson Adrienne Watson. "We stand firmly with the government and people of Israel and extend our condolences for the Israeli lives lost in these attacks."

Right to defend itself

The United States was far from alone in publicly condemning the attacks.

"I unequivocally condemn the attack carried out by Hamas terrorists against Israel," said European Union (EU) chief Ursula von der Leyen.

"It is terrorism in its most despicable form. Israel has the right to defend itself against such heinous attacks," she said.

France expressed "full solidarity with Israel and the victims" and underscored "its absolute rejection of terrorism and its commitment to Israel's security."

President Emmanuel Macron said he "firmly condemns" the attacks.

German Foreign Minister Annalena Baerbock said Israel "has our full solidarity" and "the right, guaranteed by international law, to defend itself against terrorism."

British Prime Minister Rishi Sunak said he was "shocked" by the attacks.

"Israel has an absolute right to defend itself," he said.

He went further in a post later on X (formerly Twitter), where he wrote, "As the barbarity of today's atrocities becomes clearer, we stand unequivocally with Israel."

"This attack by Hamas is cowardly and depraved."

Indian Prime Minister Narendra Modi said his country stood "in solidarity with Israel at this difficult hour."

"Deeply shocked by the news of terrorist attacks in Israel," Modi said.

EU foreign policy chief Josep Borrell said "this horrific violence must stop immediately. Terrorism and violence solve nothing. The EU expresses its solidarity with Israel in these difficult moments."

"News of civilians taken as hostages in their homes or to Gaza are appalling. This is against international law. Hostages must be released immediately," he added.

United Nations (UN) human rights chief Volker Türk said he was "deeply concerned at reports that Israeli civilians have been taken hostage."

Ukrainian President Volodymyr Zelenskyy, whose country is fending off a Russian invasion, said Israel had an "indisputable" right to defend itself.

"Terror is always a crime, not just against one country or specific victims, but against humanity as a whole," he said.

Brazil, which holds the UN Security Council presidency, also condemned the attacks on Israel.

A number of other countries, including Saudi Arabia, the United Arab Emirates (UAE), Morocco and Türkiye, lamented the loss of civilian lives following the Hamas attack.

"The UAE has expressed its sincere condolences to all the victims of the recent crisis," a UAE foreign ministry statement said, stressing the importance of protecting civilians.