Story Highlights• NEW: Israeli ambassador says "measured" response could intensify
• NEW: Buildup of Hamas military more troubling than rockets, he says
• Israeli airstrike in Gaza City kills three militants; 14 killed in 2 days
• Fighting in Gaza between Fatah, Hamas militias continues
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GAZA CITY (CNN) -- Israel's response to rockets launched from Gaza into its cities has been "measured" so far but could grow harsher if the attacks continue, Israel's ambassador to the United States said Friday.
"The basic contract between a government and its citizens is to provide them with safety," Sallai Meridor told reporters.
Israel launched more airstrikes against Hamas targets in Gaza on Friday, the second day it targeted the militant group in retaliation for dozens of rockets it says have been fired from Gaza into Israel during the last several days.
The airstrikes killed 14 Palestinians Thursday and Friday, according to Palestinian security and medical sources. (Watch as violence in Gaza intensifies )
Three Palestinian militants were killed and four wounded Friday in an airstrike on a car in the Sheikh Radwan area of Gaza City, Palestinian security sources said. The Israel Defense Forces confirmed the airstrike, saying it targeted a car carrying Hamas militants and weaponry.
Also, five militants died Friday morning south of Gaza City. An IDF spokesman said the Israeli Air Force targeted a Hamas structure that was believed to be used for Hamas meetings and that possibly concealed a tunnel near the Karni crossing in northern Gaza.
Israel said it also targeted a Hamas rocket-launching site immediately after an attack on Israel from northern Gaza.
Israel said Qassam rockets launched from Gaza hit a school and a synagogue in Sderot, near Israel's border with Gaza. Two other rockets hit a home and nearly hit a gas station, the military said. Officials said more than 90 rockets from Gaza have landed in Israel since Tuesday.
Hamas military buildup called alarming
Meridor said the buildup of Hamas' military strength in Gaza was more alarming than the rocket attacks.
"There is an ongoing buildup of Hamas, which is a terror state," he said.
Hamas is backed by Iran and Syria, he said, and is using the Egyptian border with Gaza as "an almost free passage for arms and explosives."
Meridor said Hamas is increasing its strength in Gaza to retain control over Palestinian society and jeopardize President Mahmoud Abbas' efforts to reassert authority and negotiate a peace deal with Israel.
The group's tactics, he said, were similar to those used by Hezbollah in last summer's war with Israel.
"Our response so far has been very measured because we understand the game plan of the other side and because we carefully assess the different options," Meridor said. But, he added, the situation is "very volatile."
U.S. State Department spokesman Sean McCormack noted that innocent Palestinians and Israelis caught in the crossfire are the victims of the violence, but Hamas is the culprit.
"We want to see an end to the violence, as does everybody else," McCormack said. "We are quite concerned about the Palestinian people.
"But the way to bring this about is for Hamas to start making those decisions that the international community has asked them to make -- and also to support President Abbas in his efforts to bring not only order to the Gaza Strip, but also to preserve and extend the possibilities for the Palestinian people in realizing a Palestinian state via the negotiating table."
Journalist kidnapped, released
Also on Friday, Abd Salam Abu Askar, a Palestinian journalist and chief of Abu Dhabi's Gaza bureau, was briefly kidnapped in Gaza City about 6:30 p.m. local time (11:30 a.m. ET), according to Palestinian security sources and Abu Dhabi television. He was taken from his car by Hamas militants, sources said. Abu Dhabi said he was released and returned home several hours later.
Askar has close ties with Mohammed Dahlan, the Palestinian Authority's national security adviser, and serves as one of his media consultants. Dahlan is a Fatah member and a member of the Palestinian Legislative Council.
Internal Palestinian violence between Hamas and Fatah flared again Friday when the Fatah-affiliated Presidential Guard fired rockets at Hamas-dominated Islamic University, witnesses said.
Despite this, the factional fighting was quieter Friday than two days ago, according to CNN's Ben Wedeman.
Residents of the Jabiliya refugee camp marched in the streets to show their opposition to the sectarian fighting.
CNN's Ben Wedeman and Elise Labott contributed to this report.