JERUSALEM — Gaza rocket fire struck a gas station and set it ablaze Friday in southern Israel, seriously wounding one person as rocket fire also came from Lebanon for the first time in the four-day offensive.

The explosion in Ashdod sent plumes of smoke high into the air. Israeli health officials said the blast wounded three people, including one in serious condition. Rocket fire continued in earnest from Gaza toward various locations in southern Israel.

In northern Israel, rocket fire struck near the Lebanese border and the military responded with artillery fire toward the source in southern Lebanon, military spokesman Lt. Col. Peter Lerner said.

The Lebanese military said three rockets were fired toward Israel around 6 a.m. (0300 GMT) and the Israelis retaliated by firing about 25 artillery shells on the area. Lebanese troops and United Nations peacekeepers later began searching the area, and the military said it was trying to find out who was behind the attack. No one was wounded on either side.

Southern Lebanon is a stronghold of the Shiite militant group Hezbollah, which has battled Israel numerous times. However, recent fire from Lebanon has been blamed on radical Palestinian factions in the area and Hezbollah has not been involved in the ongoing offensive. It has largely refrained from engaging Israel since a monthlong war in 2006 when it fired thousands of rockets into Israel and was pummeled by Israeli airstrikes in response.

Lerner said Israel has suspected that Lebanese militants may try to join the fray as Israel exchanges fire with Islamic Hamas militants in Gaza. However, he said it was still unclear whether Friday’s attack was “symbolic or something more substantial.”

Gaza militants already have fired more than 550 rockets against Israel in the four-day offensive. Israel’s “Iron Dome” defense system has intercepted most of those aimed at major cities but some have slipped through.

One such rocket struck the gas station in Ashdod, leaving a trail of charred vehicles in its wake. The attack looked to be the most serious in the four days of fighting that has yet to kill anyone on the Israeli side. Israeli military attacks on Gaza have killed at least 95 people, including dozens of civilians.

Frequent air raid sirens sounded across Israel on Friday, including for the first time in the northern city of Haifa. Israel has shot down at least 110 incoming rockets thus far.

Israel launched the Gaza offensive to stop incessant rocket fire against it. The military says it has hit more than 1,100 targets already, mostly what it identified as rocket-launching sites, bombarding the territory on average every five minutes.

In Gaza, an Israeli airstrike Friday hit the home of a well-known Islamic Jihad leader. Gaza health officials said five people were killed in the strike.

Lerner said the military was doing its utmost to prevent civilian casualties, calling inhabitants ahead of time to warn of an imminent attack. He said Israeli forces also fire “non-explosive munitions” at roofs as a warning and looks for people to leave before destroying a structure.

Lerner blamed Hamas for the death of innocent bystanders by firing from heavily populated areas.

Israel’s military “uses its weapons to defend its civilians. Hamas uses its civilians to defend its weapons,” he said.

With Hamas on its heels, Israeli leaders were still mulling whether to launch a ground assault in Gaza to deliver what they hope could be a decisive blow. Such a move, though, would likely involve a rise in Palestinian civilian casualties and put Israeli troops at risk as well.

During a ground incursion in early 2009, hundreds of civilians were killed and both sides drew war crimes accusations in a United Nations report

Israel has mobilized more than 30,000 reservists to supplement the potential ground operation.

Amos Yadlin, a retired general and former head of military intelligence, said Israel already re-established a deterrent factor against Hamas and should offer a cease-fire and aim to wrap up its campaign in the coming days.

“If the Israeli offer is turned down, Israel will refill its stock of legitimacy in such a way that will enable a significant expansion of the objectives and scale of the operation,” he wrote in a column published Friday in the Yediot Ahronot daily newspaper. “Hamas has taken severe blows since the start of the round of violence and has failed in almost every step it took. … However, one rocket that hits an Israeli population center will be enough to change the picture completely.”