(Adds new Israeli missile strike paragraph 10)

By Nidal al-Mughrabi

GAZA, Nov 5 (Reuters) - Hamas fired dozens of rockets at Israel on Wednesday after Israeli forces killed six Palestinian militants in an eruption of violence that disrupted a four-month-old truce along the Gaza Strip's frontier.

The rocket attacks, which included salvoes that landed in the coastal Israeli city of Ashkelon, caused no casualties. Israeli forces later killed a seventh Palestinian gunman who the military said had taken part in cross-border rocket fire.

Sources in Hamas, an Islamist group that runs Gaza, said calm could return if Israel held its fire. Israel's defence minister also signalled it did not want the truce to collapse.

A breakdown of the Egyptian-brokered Gaza ceasefire would pose yet another challenge to U.S. efforts, opposed by Hamas, to achieve a peace deal between Israel and the Palestinians.

Egypt has been working to extend the ceasefire beyond its agreed six-month duration and, in conjunction with other Arab countries, to reconcile Hamas and Palestinian President Mahmoud Abbas's secular Fatah faction at a summit in Cairo next week.

U.S. Secretary of State Condoleezza Rice arrives in the region on Thursday amid few hopes Israel and Abbas can meet Washington's target date of an agreement this year.

U.S. President-elect Barack Obama has said resolving the Israeli-Palestinian conflict would be a priority for his administration.

On Tuesday, Israeli airstrikes killed five militants and Israeli soldiers shot dead a gunman during an incursion into the Gaza Strip. Israeli forces quit the coastal enclave in 2005 and Hamas took control after routing Fatah forces two years later.

The Israeli military said the aircraft went into action after militants attacked soldiers who entered Gaza to destroy a tunnel that Hamas had planned to use to kidnap Israeli soldiers.

Hours later, an Israeli missile killed a field commander from the Palestinian faction Islamic Jihad in north Gaza. Two other gunmen were wounded. The Israeli military said the strike targeted a rocket squad that had fired across the border.

"We have no intention of violating the quiet. We have an interest in perpetuating the calm," Israeli Defence Minister Ehud Barak told reporters.

"But whenever it is necessary to thwart operations against Israel Defence Force soldiers or the civilians on the Gaza outskirts, we will act."

Hamas-led gunmen tunnelled into Israel from Gaza in June 2006 and seized Israeli soldier Gilad Shalit, who is still being held at a secret location in the territory.

Hamas has offered to release him in exchange for hundreds of Palestinians in Israeli prisons, including militants who killed Israelis. Israel has not agreed to the terms.

"We are all exploiting the calm, and its extension, in order to find the way to return Gilad Shalit home," Barak said.

(Writing by Joseph Nasr; Editing by Michael Roddy)

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