* Raid appears to be first ground action of offensive
* Israel says urges civilians to evacuate Gaza rocket sites
* Gaza death toll reaches 149, includes many civilians
* Foreign pressure mounts for ceasefire (Adds details of naval raid, plans to seek Gaza civilian evacuation in northern Gaza)
By Nidal al-Mughrabi and Ari Rabinovitch
GAZA/JERUSALEM, July 13 (Reuters) - Israeli naval commandos clashed with Hamas militants in a raid on the coast of the Gaza Strip on Sunday, in what appeared to be the first ground assault of a six-day Israeli offensive on the territory aimed at stopping Palestinian rocket fire.
With aerial support from fighter jets, the Israeli force attacked a site in northern Gaza used to launch long-range rockets, an Israeli military spokesman, Lieutenant Colonel Peter Lerner, said. Militants opened fire, wounding four commandos, but the launch site was hit, he said.
Hamas said its fighters had fired at the Israeli force offshore, preventing them from landing.
Lerner said the forces had "completed their mission" and that the results of the raid "would be the first published ground activity" by naval troops in Gaza, in an offensive that Palestinian officials said has killed 149 Palestinians, many of them civilians.
No Israeli has been killed by Hamas's rocket fire.
Two hours after the announcement of the raid at 0100 GMT, militants fired a long-range rocket salvo, with air raid sirens sounding at Tel Aviv's Ben-Gurion airport and some city suburbs.
Israel says a ground invasion of Gaza remains an option, and it has already mobilised about 20,000 reservists to do so, but most attacks have so far been from the air, hitting some 1,200 targets in the territory.
Lerner said Israel would drop leaflets in northern Gaza's Beit Lahiya to urge thousands of residents to evacuate ahead of planned strikes there.
"It's our assessment a huge majority of rockets are being launched (at Israel) from northern areas" in Gaza, he said.
Islamist Hamas, which dominates Gaza, has fired hundreds of rockets into Israel, striking the deepest inside the country ever.
The Israeli military said more than 800 rockets had been launched since its offensive began on Tuesday. Israel said it has carried out 1,320 attacks on militant targets, half against rocket launch sites, and the rest at command centres, rocket manufacturing installations, warehouses and smuggling tunnels.
The cross-border violence shows no signs of abating despite mounting international pressure on both sides to end the violence. The U.N. Security Council called for a cessation of hostilities and Western Foreign Ministers were due to meet on Sunday to discuss the need for a ceasefire.
Sirens warning of incoming rockets went off throughout the night in Israel, sending residents running for safe rooms and bomb shelters.
Israeli aircraft carried out a series of attacks in Gaza, including against a police headquarters and a security compound, Palestinian officials said.
GAZA CIVILIANS KILLED
A woman and a three-year-old girl were killed in the air strikes in the early hours of Sunday morning, Palestinian officials said.
An Israeli air strike on the home of Gaza's police chief killed 18 people on Saturday, Gaza's health ministry said, in what was the single deadliest attack of the offensive, while Hamas fired its largest salvo of rockets yet on Tel Aviv.
A Hamas source said the police chief, Tayseer Al-Batsh, was in critical condition. All of those killed in the air strike which television footage showed was reduced to piles of rubble, were members of Al-Batsh's family.
Ashraf Al-Qidra, spokesman for the Gaza Health ministry, said 45 people were wounded and others were still trapped under the rubble where rescue workers were searching.
Israel says it tries to avoid civilian casualties and accuses of Hamas of putting innocent Gazans in harm's way by placing weaponry and gunmen in residential areas.
A senior Israeli military officer said aircraft had aborted "hundreds" of strikes to avoid collateral damage and that targets bombed were meant to impact Hamas fire capacity.
Many of the rockets have been shot down above Israeli towns by Iron Dome, a partly U.S.-funded interceptor system. Israel rushed an eighth Iron Dome into service on Saturday to counter stronger-than-expected rocket fire from Gaza.
Israel said it intercepted nine of 131 rockets launched at central areas, sending hundreds of thousands of its civilians running for cover in the south and in Tel Aviv. Nineteen rockets crashed into open areas.
Fire was also exchanged across Israel's northern border.
Rockets fired late on Saturday from Lebanon hit Israel, and the military said it responded with artillery fire.
Southern Lebanon is a stronghold of Hezbollah, a Shi'ite Muslim group that battled Israel seven years ago and is engaged in Syria's civil war in support of President Bashar al-Assad; but there are also Palestinian groups in the same area.
Israel believes a Palestinian group was behind that rocket attack and not Hezbollah, Lerner said. Hamas claimed responsibility for the rocket fire from Lebanon.
(Editing by Louise Ireland)