* 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 Putin, Guterres, Macron, updates Gaza toll )
By Nidal al-Mughrabi and Rami Ayyub
GAZA/JERUSALEM, May 13 (Reuters) - Palestinian militants fired more rockets into Israel's commercial heartland on Thursday as Israel kept up a punishing bombing campaign in Gaza and massed tanks and troops on the enclave's border.
The four days of cross-border violence showed no sign of abating and Israeli Prime Minister Benjamin Netanyahu said the campaign "will take more time".
The violence has also spread to mixed communities of Jews and Arabs in Israel, a new front in the long conflict.
Synagogues were attacked and fighting broke out on the streets of some communities, prompting Israel's president to warn of the danger of civil war.
Worried that the region's worst hostilities in years could spiral out of control, the United States is sending an envoy, Hady Amr. Truce efforts by Egypt, Qatar and the United Nations have so far offered no sign of progress.
In renewed air strikes on Gaza, Israeli warplanes struck a six-storey residential building that it said belonged to Hamas, the Islamist group that controls the Palestinian enclave. Netanyahu said Israel has struck close to a thousand militant targets in Gaza in total.
The Gaza health ministry said 87 people, including 18 children and eight women, have been killed and 530 others wounded - 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.
A Palestinian rocket had earlier crashed into a building near Israel's commercial capital of Tel Aviv, injuring five Israelis, police said. Seven people have been killed in Israel since hostilities began, the Israeli military said.
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.
Hamas armed wing spokesman Abu Ubaida responded to the troop buildup with defiance, urging Palestinians to rise up.
"Mass up as you wish, from the sea, land and sky. We have prepared for your kinds of deaths that would make you curse yourselves," he said.
Health authorities in Gaza said they were investigating the deaths of several people overnight who they said may have inhaled poisonous gas.
U.S. President Joe Biden said he hoped fighting "will be closing down" sooner rather than later.
Russian President Vladimir Putin and United Nations Secretary-General Antonio Guterres appealed in a video call for an end to the fighting.
"The main goal is to stop violent acts from both sides and ensure the safety of the civilian population," the Kremlin said in a statement.
French President Emmanuel Macron also urged a "definite reset" of negotiations between the two sides, and British Prime Minister Boris Johnson called for an "urgent de-escalation" of violence.
Prime Minister Benjamin Netanyahu has said Israel will continue strikes against 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 which Israel said were linked to the faction's activities.
Israel unleashed 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.
The hostilities have opened a new front by fuelling tension between Israeli Jews and the country's 21% Arab minority who live alongside them in some communities.
Jewish and Arab groups attacked people and damaged shops, hotels and cars overnight. In Bat Yam, south of Tel Aviv, dozens of Jews beat and kicked a man thought to be an Arab as he lay on the ground.
One person was shot and badly wounded by Arabs in the town of Lod, where authorities imposed a curfew, and over 150 arrests were made 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," he said.
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.
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, including Washington's recognition of disputed Jerusalem as Israel's capital.
(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 Timothy Heritage and Angus MacSwan)