* Israel massing troops on Gaza border recalls 2014 war
* Hundreds of rockets hit Israel, its jets strike Gaza
* Escalation follows weeks of tension in Jerusalem
* U.S., United Nations focused on de-escalation (Adds details, quotes)
By Nidal al-Mughrabi and Rami Ayyub
GAZA/JERUSALEM, May 13 (Reuters) - Israeli troops massed at Gaza's border on Thursday and Palestinian militants pounded Israel with rockets in intense hostilities that have caused international concern and touched off clashes between Jews and Arabs in Israel.
Days of violence between Jewish Israelis and the country's Arab minority worsened overnight, with synagogues attacked and fighting breaking out on the streets of some communities.
With concern growing that the violence that flared on Monday could spiral out of control, the United States is sending an envoy, Hady Amr, to the region. But efforts to end the worst hostilities in years appear so far to have made no progress.
In renewed air strikes on Gaza, Israel struck a six-storey residential building in Gaza City that it said belonged to Hamas, the Islamist group that controls the Palestinian enclave.
At least 83 people have been killed in Gaza since violence escalated on Monday, medics said, further straining hospitals already under heavy pressure during the COVID-19 pandemic.
"We are facing Israel and Covid-19. We are in between two enemies," said Asad Karam, 20, a construction worker, standing beside a road damaged during the air strikes. An electricity pole had collapsed by the road, its wires severed.
In the latest Palestinian rocket attacks, one rocket crashed into a building near Israel's commercial capital of Tel Aviv, injuring five Israelis, police said. Sirens blared in cities across southern Israel, sending thousands running for shelters.
Seven people have been killed in Israel, its military said.
"All of Israel is under attack. It's a very scary situation to be in," said Margo Aronovic, a 26-year-old student, in Tel Aviv.
Israel has prepared combat troops along the Gaza border and was in "various stages of preparing ground operations", a military spokesman said, a move that would recall similar incursions during Israel-Gaza wars in 2014 and 2008-2009.
Health authorities in Gaza said they were investigating the deaths of several people overnight who they said may have inhaled poisonous gas. Samples were being examined and they had yet to draw any final conclusions, they said.
U.S. President Joe Biden said he hoped fighting "will be closing down sooner than later". A British minister urged Israel and Hamas to "take a step back" from the escalation.
Israeli Prime Minister Benjamin Netanyahu has vowed to "continue acting to strike at the military capabilities of Hamas" and other Gaza groups. Hamas is regarded as a terrorist group by the United States and Israel.
On Wednesday, Israeli forces killed a senior Hamas commander and bombed several buildings, including high-rises and a bank, which Israel said was linked to the faction's activities.
Hamas signalled defiance, with its leader, Ismail Haniyeh, saying: "The confrontation with the enemy is open-ended."
Israel launched its offensive after Hamas fired rockets at Jerusalem and Tel Aviv in retaliation for Israeli police clashes with Palestinians near al-Aqsa mosque in East Jerusalem during the Muslim fasting month of Ramadan.
Turkey, whose hosting of Hamas leaders in Istanbul in recent years has contributed to a falling out with Israel, called on Muslim countries to show a united and clear stance over the Israel-Gaza violence.
In the fighting inside Israel, where some in the 21% Arab minority have mounted violent pro-Palestinian protests, attacks by Jews on Arabs passing by in ethnically mixed areas have worsened.
One person was in critical condition after being shot by Arabs in the Arab-Jewish town of Lod, where authorities imposed a curfew, police said.
Over 150 arrests were made overnight in Lod and Arab towns in northern Israel, police said.
Israeli President Reuven Rivlin called for an end to "this madness".
"We are endangered by rockets that are being launched at our citizens and streets, and we are busying ourselves with a senseless civil war among ourselves," said the president, whose role is largely ceremonial.
A number of foreign carriers have cancelled flights to Israel because of the unrest.
The fatalities in Israel include a soldier killed while patrolling the Gaza border and six civilians, including two children and an Indian worker, medical authorities said.
Gaza's health ministry said 17 of the people killed in the enclave were children and seven were women. The Israeli military said some 400 of 1,600 rockets fired by Gaza factions had fallen short, potentially causing some Palestinian civilian casualties.
The conflict has led to the freezing of talks by Netanyahu's opponents on forming a governing coalition to unseat him after Israel's inconclusive March 23 election.
Although the latest problems in Jerusalem were the immediate trigger for hostilities, Palestinians are frustrated by setbacks to their aspirations for an independent state in recent years.
These include Washington's recognition of disputed Jerusalem as Israel's capital, a U.S. plan to end the conflict that they saw as favourable to Israel and settlement building.
(Reporting by Nidal al-Mughrabi, Rami Ayyub and Dan Williams, Additional reporting by Nandita Bose and Steve Holland in Washington and Michelle Nichols in New York; Editing by Howard Goller, Michael Perry and Timothy Heritage)
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