Gaza militants pummelled Israel with dozens of rockets and mortars and Israeli airstrikes killed two Palestinians in a sharp escalation of violence following a landmark visit to the coastal territory by the leader of Qatar.
Hostilities have been simmering for weeks, but erupted into barrages from Gaza immediately after the Qatari ruler left the territory. Militants from the ruling Hamas movement joined the fray, undercutting the emir's appeal to avoid confrontation with Israel.
Israeli leaders vowed that their country would not reconcile itself to attacks from the coastal strip.
"We didn't ask for this escalation and didn't initiate it," prime minister Benjamin Netanyahu said after touring a missile defence battery. "But if it continues, we are prepared to embark on a far more extensive and penetrating operation."
Asked if Israel was considering a ground operation in the Palestinian territory, defence minister Ehud Barak said that "if we need a ground operation there will be a ground operation. We will do whatever necessary to stop this wave" of violence.
The Israeli military said 72 rockets and mortars landed in Israel and that Israeli aircraft struck Gaza four times.
Hamas' military wing and a smaller militant faction claimed credit for the rocket and mortar fire.
The deaths brought to four the number of Palestinians who have died in strikes on Gaza in the past two days. Two foreign workers in Israel were critically wounded in the rocket fire and some militants were injured in the Israeli air attacks. One of the rockets hit a house, causing no injuries, and one of the airstrikes struck a mosque in the southern Gaza village of Khouza for the second time in several weeks.
Hamas has largely stayed out of direct confrontation with Israel since a major Israeli offensive in Gaza four years ago inflicted heavy casualties and damage, and halted much of the rocket fire. But it is also under pressure from various militant groups, including al Qaida-inspired Salafis active in Gaza, to prove it remains in confrontation with Israel, whose existence it rejects.
The Qatari emir, Sheik Hamad bin Khalifa Al Thani, had urged the Iranian-backed Hamas to do everything possible to avoid violence with Israel. But the emir's visit and promise of aid bolstered Hamas' flagging popularity and might have encouraged it to join the latest round of hostilities, which had previously involved smaller militant groups.