Fighting intensified as Israel continued its Gaza offensive for a fourth day Friday, with Palestinian militants continuing rocket attacks on the Jewish state and rocket fire coming from Lebanon for the first time. 
A rocket fired from Lebanon into northern Israel early Friday drew retaliatory artillery fire from Israeli troops.  The rocket, launched around 6 a.m. local time, struck an open area near Metula at Israel's northernmost tip without causing casualties or damage, the AFP news agency reported. It said Israeli army spokesman Lieutenant Colonel Peter Lerner said artillery units had fired a barrage at "suspicious positions" over the border.

Israeli troops responded by firing about 25 artillery shells on southern Lebanon, the Associated Press reported.  Military officials, speaking on Israeli public radio, said they believed the attack was carried out by a small Palestinian group aligned with Islamist Hamas militants, AFP reported. Israel filed a complaint with the United Nations Interim Force in Lebanon, which monitors the countries' border, AFP reported. 
Hamas issues warning 
Also Friday, Hamas’ armed wing warned airlines against using Tel Aviv's international airport. “The armed wing of the Hamas movement has decided to respond  to the Israeli aggression, and we warn you against carrying out flights to Ben-Gurion airport, which will be one of our targets today because it also hosts a military air base,'' the Islamist group's Izz el-Deen al-Qassam  Brigades said in a statement, Reuters reported. The group, saying it wanted to avoid injuries to passengers, had fired at least one rocket toward the airport Friday. Airport activity has continued despite the aerial offensive, which Israel launched Tuesday. The airport quieted for about 10 minutes following a siren that had sounded because of a general alert in the Tel Aviv area, a spokesman for Israel’s Airports Authority told Reuters.
Obama expresses concern 
The ongoing strife prompted U.S. President Barack Obama to phone Israeli Prime Minister Benjamin Netanyahu Thursday to express concern and offer help in negotiating a resolution to the crisis. "The United States remains prepared to facilitate a cessation of hostilities, including a return to the November 2012 cease-fire agreement," the White House said Obama told Netanyahu, according to Reuters news agency. The president also repeated his condemnation of Hamas rocket attacks on Israel and reaffirmed Israel’s right to defend itself.

Meanwhile, U.S. Secretary of State John Kerry phoned Egypt’s foreign secretary to ask that the country exert its influence in defusing the situation, Reuters reported. Egypt played a critical role in mediating a cease-fire between Israel and Hamas in 2012. The U.S. government refuses to negotiate directly with Hamas, which Washington considers a terrorist organization. State Department spokeswoman Jen Psaki said Kerry “has been reaching out to countries in the region, including Qatar, including Egypt. … Any country in the region that can play a role in bringing an end to the rocket fire from Hamas, we're certainly going to be engaged with."
UN urges restraint 
United Nations chief Ban Ki-moon called Thursday for an immediate cease-fire in the Israeli conflict with Hamas militants in Gaza, but neither of the warring sides showed any hint of stopping its attacks.

The U.N. secretary-general told an emergency meeting of the Security Council that Israel and Hamas "must exercise maximum restraint" to end the fighting. As Israeli warplanes relentlessly attacked hundreds of Hamas targets in Gaza Thursday, Netanyahu told a parliamentary committee that a cease-fire is "not even on the agenda." Hamas continued to fire rockets at the key Israeli cities of Tel Aviv and Jerusalem, although the Jewish state said its Iron Dome missile defense system has knocked down most of them. Israel has reported no casualties, while Palestinian officials said Thursday the death toll in Gaza from the Israeli assault has reached at least 85 militants and civilians, including women and children. The Israeli military said its air strikes had hit another 320 Hamas targets Thursday, focusing on underground tunnel networks and rocket launching sites.
'Genocide' accusation
Palestinian President Mahmoud Abbas accused Israel of "genocide" in the attacks. Pillars of flames and black smoke filled the skies over Gaza. Israeli officials said the air attacks would continue as long as Hamas continues to fire rockets into Israel. Air raid sirens sounded in cities and some Israelis knelt along roadsides, their arms covering their heads.Turkish Prime Minister Tayyip Erdogan said Israeli bombardment of Gaza is blocking efforts to normalize relations between the two countries. Turkey was once Israel's closest regional ally, but those ties were frayed by a 2010 attack on a vessel taking part in an aid flotilla challenging Israel's naval blockade of the Gaza Strip. Ten people were killed when Israeli commandos stormed the Mavi Marmara.  Violence in the region has escalated since the killing of three Jewish students last month and the murder of a Palestinian teen in a suspected revenge attack