(CNN) -- As dozens of Palestinian rockets rained down on southern Israel, the country's air force carried out new raids on positions in Gaza on Wednesday. Two days of airstrikes have killed four Palestinians, at least two of them identified as militants.
Attacks and counterattacks between the two sides are a frequent occurrence but appear to have escalated in the past two days.
On Tuesday night, the Israeli military said its aircraft had targeted two rocket-launching squads in northern Gaza as they prepared to fire into southern Israel. That attack killed two militants from Al-Qassam, the military wing of Hamas, and wounded two others.
But the barrage of rockets continued, with more than 65 fired from Gaza in an eight-hour period, the military said Wednesday morning.
Israeli aircraft struck Gaza again early Wednesday, killing two people and seriously wounding nine others, said Ashraf al-Qidra, the spokesman for the health ministry in Gaza.
The strike targeted a rocket-launching site in northern Gaza, the Israel Defense Forces said, adding that its tanks had also shelled "terrorist targets."
The Palestinian rocket attacks wounded three Thai workers in Israel, hospital officials said.
Israeli areas across Gaza are under heightened alert amid fears that more rockets could be launched.
Israeli Prime Minister Benjamin Netanyahu said Wednesday that Israel "neither chose nor initiated this escalation, but if it continues, we are prepared for much more extensive and deeper action. In any case, we will continue with preventative operations. Whoever intends to attack Israeli citizens needs to know that he will bear the consequences."
Defense Minister Ehud Barak toured areas hit by rockets in southern Israel and said the military "is working efficiently against all the terror elements. Since the beginning of the month, 15 active terrorists have been killed in the act, and more have been injured."
He wished a speedy recovery to those injured by rockets fired from Gaza. "Hamas will receive its punishment for what has happened here," he vowed, promising that Israel will "work with patience and level-headedness."
"There is no way to eliminate the hatred or hostility entrenched in Hamas. But the IDF will act on the border, from the air and in any other necessary way in order to strike down those who attempt to cause us harm," he said.
The latest violence came after the emir of Qatar, Sheikh Hamad bin Khalifa al-Thani, visited Gaza on Tuesday, becoming the first head of state to visit since Egypt and Israel instituted a blockade in 2007.
Annual Qatari aid to Palestinians in Gaza will increase from $250 million to $400 million, Hamas Prime Minister Ismail Haniya said in Khan Younis in southern Gaza.
In Washington, State Department spokeswoman Victoria Nuland said U.S. officials "share Qatar's deep concern for the welfare of the Palestinian people, including those residing in Gaza."
But she expressed continuing concern over the "destabilizing role" of Hamas, which has governed Gaza since 2007.
CNN's Michael Schwartz and Kareem Khadder contributed to this report.