(Adds army comment, Qurie comment)
GAZA, Jan 15 (Reuters) - Israel killed at least 18 Palestinians, most of them Hamas militants, in the Gaza Strip on Tuesday, in violence the Palestinian Authority said was a "slap in the face" to U.S. President George W. Bush's peace efforts.
A volunteer from Ecuador, working on an Israeli kibbutz, or farming community, bordering the Hamas-controlled Gaza Strip, was killed by a Palestinian sniper near the frontier fence. Hamas claimed responsibility for shooting the man.
The violence, four days after Bush ended a visit to Israel and the occupied West Bank, resulted in the highest number of Palestinians killed in a single day since late 2006.
An army spokesman said troops and armoured forces backed by aircraft pushed 2 km (1.2 miles) into the coastal strip to stop militants from firing rockets and mortars into Israel.
In a statement, the West Bank-based Palestinian government said Israel's "ugly crimes were a slap in the face" to efforts by Bush and the international community to resume peacemaking that would lead to the creation of a Palestinian state.
"There was a massacre today against our people, and we say to the world that our people will not remain silent against such crimes," said Abbas.
Chief Palestinian negotiator Ahmed Qurie said Israel's raids created "obstacles and difficulties for negotiations".
Hamas's armed wing claimed responsibility for firing rockets into Israel on Tuesday for the first time since the Islamist group routed Palestinian President Mahmoud Abbas's Fatah forces to take control of the Gaza Strip in June.
Hamas had fired mortar bombs and did not try to stop other militant groups from firing rockets into Israel, but the claim of attacks marked a change of approach toward the Jewish state.
Medical workers and Hamas said 18 Palestinians -- 13 Hamas men, three other militants and two civilians -- were killed in fighting with Israeli forces and a series of missile strikes in the northern and eastern Gaza Strip. One of those killed was a son of Mahmoud al-Zahar, a local Hamas leader.
Ambulance workers said at least five Israelis were wounded by the rocket fire.
ROCKETS AND RAIDS
Israeli President Shimon Peres said as long as militants continued to fire rockets into the Jewish state, "we are left without a choice but to answer and stop it".
White House spokesman Gordon Johndroe, speaking during Bush's visit to Riyadh, said: "There are clearly extremists who are trying to derail the peace process. We hope that the Israelis will be targeted in their strikes against the militants and we urge the Palestinians to stop killing innocent Israelis."
Hamas Islamists oppose U.S.-backed peace talks between Israel and the Palestinian Authority. Bush predicted during his three-day visit that a peace treaty would be signed before he left office in January 2009, despite deep public scepticism.
"This is one of the results of the Bush visit. He encouraged the Israelis to kill our people," said Zahar as he viewed the body of his son at a Gaza hospital.
"We will pursue the path of liberation, complete liberation, even if all of us are killed," Zahar said.
Israel, which pulled troops and settlers out of the Gaza Strip in 2005, frequently mounts raids in the territory to try to halt rocket attacks disrupting life in border communities.
At least 40 Palestinians were wounded, five of them critically, in the fighting, hospital workers said. Gaza's Shifa hospital issued an appeal for blood donations.
Newly appointed U.N. Middle East envoy Robert Serry on his first visit to Gaza condemned both sides for the violence and said the situation in the coastal territory was unsustainable.
"I really appeal to all parties to end the violence and to find ways to deescalate the present escalation of violence," Serry said.
The United States and Israel have moved to isolate Hamas over its refusal to recognise Israel, renounce violence and accept existing Israeli-Palestinian interim peace deals. (Additional reporting by Ori Lewis and Ari Rabinovitch in Jerusalem, Haitham Tamimi in Hebron and Ali Sawafta and Mohammed Assadi in Ramallah; Writing by Jeffrey Heller in Jerusalem; Editing by Caroline Drees)
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