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TEL AVIV — Israeli airstrikes hit several targets in the Gaza Strip on Friday, killing a number of people including a senior commander of a Palestinian militant group, in the deadliest escalation of violence in the territory since an 11-day war last year.

The strikes hit residential apartments as well as militant watchtowers, killing at least 10 people including a 5-year-old girl, and wounding 55, the Gaza health ministry said. One airstrike killed Taysir al-Jabari, a senior military leader in Islamic Jihad, the second-largest militant group in Gaza, according to both Islamic Jihad and the Israeli military.

The escalation followed one of the least violent phases in Gaza for several years. After the war in May 2021, there were relatively few cross-border exchanges of fire, as tensions shifted to the occupied West Bank. Both Israel and Hamas, the militant group that dominates Gaza, had signaled they sought to avoid another full-scale war over the enclave, which has been under an Israeli and Egyptian blockade since 2007.

But the violence on Friday drew warnings of retaliation from Palestinian militant groups and raised the possibility of extended conflict.

The airstrikes followed almost a week of rising tensions between Israel and Islamic Jihad, which often acts independently of Hamas. Israel arrested one of the group’s senior commanders in the West Bank this week, leading to threats of reprisal from its Gaza leadership.

Israel said that Islamic Jihad had been poised to respond with a revenge attack, and that it had targeted Mr. al-Jabari and others pre-emptively.

“Israel will not allow the terrorist organizations to set the agenda in the Gaza Strip and threaten the citizens of the State of Israel,” Prime Minister Yair Lapid of Israel said shortly after the attack.

Israel closed crossings into the Gaza Strip this week in anticipation of a retaliatory attack following the arrest in the West Bank.

After the airstrikes, Islamic Jihad said it would respond with force, and cities in southern Israel opened bomb shelters in anticipation of rocket fire from Gaza.

“The enemy has started a war targeting our people and we all need to defend ourselves and our people,” an Islamic Jihad statement said.

Hours later, Palestinian militants fired scores of rockets into Israeli airspace, and Islamic Jihad claimed responsibility. There were no immediate reports of damage or casualties.

It was not immediately clear whether Hamas would join Islamic Jihad in responding. In the past, Hamas has occasionally sat on the sidelines as Islamic Jihad clashed with Israel.

“As we mourn the leader al-Jabari and the righteous martyrs, we affirm that matters are open to all directions, calling for an end to the Zionist aggression against our people,” Ismail Haniyeh, chief of Hamas’s political bureau, said in a statement.

Plumes of smoke billowed over the Gazan skyline and on the ground, throngs of rescuers, medics and onlookers gathered in the street near where the Islamic Jihad commander had been killed. Photographs posted online showed him being carried through a crowd, and a grieving man carrying what appeared to be a dead child covered in a shroud.

The airstrikes shifted the focus of conflict back to Gaza. Since March, Palestinian attackers have killed at least 19 Israelis and foreigners in the West Bank and Israel, in the most intense wave of stabbings and shootings for several years. In response, Israel has mounted almost nightly raids in the West Bank, arresting hundreds of Palestinians and killing more than 40, according to the United Nations.

Several civilians have been caught in the violence, including Shireen Abu Akleh, a Palestinian American broadcaster shot dead while covering an Israeli raid in May.

Fady Hanona and Iyad Abu Hweila contributed reporting from Gaza City.