* Israel has hinted of possible ground invasion into Gaza
* 85 Palestinians, many civilians, killed in Israeli strikes
* Gaza militants fire rocket salvoes deep into Israel
* Obama tells Netanyahu U.S. can facilitate truce talks
* Rockets from Lebanon land in northern Israel (Updates casualty figures)
By Nidal al-Mughrabi and Ori Lewis
GAZA/JERUSALEM July 11 (Reuters) - Israeli air strikes on Gaza killed four more Palestinians before dawn on Friday, raising the death toll from the four day offensive to at least 85, while a Palestinian rocket hit a fuel tanker at an Israeli petrol station causing a huge blaze.
Israeli leaders, determined to end Palestinian rocket attacks deep into the Jewish state, have hinted that they could order the first ground invasion of the coastal strip in five years. Some 20,000 army reservists have been mobilised.
The Israeli military said it launched fresh naval and air strikes early on Friday, giving no further details.
An air strike on a house in Gaza City killed a man described by Palestinian officials as a doctor and pharmacist. Medics and residents said an Israeli aircraft also bombed a three-storey house in the southern town of Rafah, killing three people.
The salvoes into Israel have so far caused no fatalities, due in part to interception by Israel's partly-U.S. funded Iron Dome aerial defence system.
However, eight people were wounded, one in serious condition, by a rocket on Friday that hit a fuel tanker at a petrol station in Israel's port city of Ashdod, an ambulance service spokesman said. Firefighters doused the blaze.
Three rockets were intercepted over the Tel Aviv metropolitan area on Friday morning, the military said.
Fire was also exchanged across Israel's northern border. Lebanese security sources said two rockets were fired into northern Israel on Friday but they did not know who had fired them. Israel responded with artillery fire.
Israel's chief military spokesman, Brigadier-General Motti Almoz, said one rocket fell near Kibbutz Kfar Yuval and that there were no casualties or damage.
The Palestinians said Israeli tanks fired shells east of Rafah, naval forces sent shells into a security compound in Gaza City and aircraft bombed positions near the borders with Egypt and Israel.
Prime Minister Benjamin Netanyahu said in a televised statement on Thursday: "So far the battle is progressing as planned, but we can expect further stages in future. Up to now, we have hit Hamas and the terror organisations hard and as the battle continues we will increase strikes at them."
The four day offensive is the deadliest since October 2012, when around 160 Palestinians and six Israelis were killed during an Israeli campaign to punish Hamas for missile attacks. That conflict was eventually halted with Egyptian mediation.
If Israel launches a ground invasion of Gaza, it would be the first since a war in early 2009 when 1,400 Palestinians and 13 Israelis were killed.
"We have long days of fighting ahead of us," Defence Minister Moshe Yaalon said.
Hamas spokesman Sami Abu Zuhri sounded a defiant note, when asked about Yaalon's remarks. "Our backs are to the wall and we have nothing to lose," he said. "We are ready to battle until the end."
The Israeli military said some 550 projectiles have been fired at Israel since Tuesday by Islamist group Hamas, the dominant force in Gaza, and by other militant groups.
Some rockets have landed more than 100 km (60 miles) from Gaza. Sirens sounded as far north as the Israeli city of Haifa on Friday, though police said no remnants of rockets, which Hamas said it had fired, were found.
The military added that it had targeted some 210 sites in the Gaza Strip over the past 24 hours, among them "long-range rocket launchers, Hamas leadership facilities and terror and smuggling tunnels".
CHILDREN AMONG THE DEAD
Medical officials in Gaza said at least 70 civilians, including children, were among those killed since the offensive began on Tuesday.
U.S. President Barack Obama told Netanyahu by telephone on Thursday that the United States was willing to help negotiate a ceasefire, the White House said. A spokeswoman for U.S. Secretary of State John Kerry said: "Nobody wants to see a ground invasion."
French President Francois Hollande voiced his concern at the civilian deaths and called for a truce. Hollande and Kerry both spoke to Palestinian Authority President Mahmoud Abbas, who is based in the West Bank and entered a power-sharing deal with Hamas in April after years of feuding.
Kerry also spoke to Egypt's foreign minister in an attempt to get Cairo to use its influence to calm the situation, State Department spokeswoman Jen Psaki said. Kerry, she said, had also "reached out" to Qatar.
Israel's offensive followed a build-up in violence after three Israeli students were killed in the occupied West Bank last month and a Palestinian youth was killed in a suspected revenge attack in Jerusalem.
Palestinian rocket fire escalated after Israeli forces arrested hundreds of Hamas activists in the West Bank while searching for the youths, who Israel said were abducted and killed by Hamas.
Israel says it has struck more than 1,000 targets in Gaza from the air and the sea, including militant commanders' homes, which it describes as command and control centres.
Hamas says at least 200 houses have been bombed by Israel since Tuesday and that most of the fatalities have come as a result of the house bombings.
Owners of some of the targeted homes have received telephoned warnings from Israel to get out. In other cases, so-called "knock-on-the-door" missiles, which do not carry explosive warheads, were first fired as a signal to evacuate. Scenes of families fleeing their homes have played out daily.
Residents said in Friday's attack in Rafah no warning was issued and the victims were asleep when their house was bombed.
(Additional reporting by Sylvia Westall in Beirut; Writing by Maayan Lubell and Ori Lewis; Editing by Peter Graff)