* Efforts proceed to secure permanent ceasefire
* Israel keeps focus on militant tunnels in Gaza
* Tensions high in West Bank after deadly clashes
* 883 Palestinians killed in Gaza since July 8 (Adds 18 members of family killed, Gazans take to streets minutes after truce)
By Nidal al-Mughrabi and Maayan Lubell
GAZA/JERUSALEM, July 26 (Reuters) - A 12-hour humanitarian truce went into effect on Saturday after Israel and Palestinian militant groups in the Gaza Strip agreed to a U.N. request for a pause in fighting and efforts proceeded to secure a long-term cease-fire moved ahead.
The Gaza Health Ministry said 18 members of a single family were killed by Israeli tank shelling in the southern Gaza Strip shortly before the truce took effect at 8 a.m. (0500 GMT). An Israeli military spokeswoman said she was checking the report.
Israel's military said it would hold fire for 12 hours, but would press on with its search for tunnels used by militants. A spokesman for the Islamist group Hamas, which dominates Gaza, said all Palestinian factions would abide by the brief truce.
Hundreds of Palestinians poured into the streets in the minutes after the truce took force, some headed on foot to their homes to inspect damage from the Israeli assaults and many lined up outside banks to withdraw cash and stock up on supplies.
Fighting had continued overnight as U.S. Secretary of State John Kerry, on a visit to the region, spearheaded international efforts to end 19 days of conflict in which 883 Palestinians, many of them civilians, have been killed.
Gaza officials said five people were killed in Israeli air strikes in the night and militants fired a barrage of rockets out of Gaza, triggering sirens across much of southern and central Israel. No injuries were reported, with the Iron Dome interceptor system shooting down some of the projectiles.
Gaza Health Ministry spokesman Ashraf Al-Qidra said that 18 members of the Al-Najar family killed had been trapped inside their house in Khuzaa village east of Khan Younis since Thursday and that many others were wounded in Israeli tank shelling.
Israel said that two more of its soldiers were killed in Gaza, bringing the army death toll to 37, as troops battled militants in the north, east and south of Gaza - a tiny Mediterranean enclave that is home to 1.8 million Palestinians.
It also announced that a soldier unaccounted for after an ambush in Gaza six days ago was definitely dead, although his body had not been recovered. Hamas said on Sunday it had captured the man but did not release a photograph of him.
Three civilians have also been killed in Israel by rockets from Gaza - the kind of attack that surged last month amid Hamas's anger at a crackdown on its activists in the West Bank, prompting the July 8 launch of the Israeli offensive.
Diplomats will pursue efforts to secure a ceasefire in Paris on Saturday, with France hosting officials from the United States, Britain, Germany, Italy, the European Union, Turkey and Qatar, a French diplomatic source said.
Israel on Friday rejected international proposals for an extended ceasefire, a government source said, but Kerry, speaking in Cairo, said no formal proposals had yet been put forward.
The top U.S. diplomat said there were still disagreements on the terminology, but he was confident there was a framework that would ultimately succeed and that "serious progress" had been made, although there was more work to do.
Hamas wants an end to an Israeli-Egyptian blockade of Gaza before agreeing to halt hostilities. Israeli officials said any ceasefire must allow the military to carry on hunting down Hamas's tunnel network that criss-crosses the Gaza border.
Israel says some of the tunnels reach into Israel and are meant to carry out attack on Israelis. Other underground passages serve as weapon caches and serve as Hamas bunkers.
The Gaza turmoil has stoked tensions in east Jerusalem and the occupied West Bank, some of which U.S.-backed Palestinian President Mahmoud Abbas governs in uneasy coordination with Israel.
Medics said eight Palestinians were killed in incidents near the cities of Nablus and Hebron on Friday, including one shooting that witnesses blamed on an apparent Jewish settler.
On Thursday night, 10,000 demonstrators marched in solidarity with Gaza near the Palestinian administrative capital Ramallah - a scale recalling mass revolts of the past.
Protesters surged against an Israeli army checkpoint, throwing rocks and Molotov cocktails, and Palestinian medics said one was shot dead and 200 wounded when troops opened fire.
Abbas's Palestine Liberation Organization called for more demonstrations in the West Bank and said it was at the same time working to secure a cease-fire deal. (Editing by Ken Wills and Ron Popeski)