Story Highlights• NEW: Fighting leaves at least 12 dead, Fatah presidential guards among them
• Fatah and Hamas clash sporadically Tuesday on streets of Gaza City
• Member of Hamas' military wing reportedly killed Tuesday
• Violence erupts just hours after Palestinian spokesman announces truce
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(CNN) -- For the second consecutive day, a cease-fire between rival Palestinian militias was shattered as the factions clashed in Gaza on Tuesday, with at least 12 dead in related violence.
Hamas gunmen fired on a Palestinian presidential guards post at the Karni border crossing with Israel, killing at least seven members of the guard, Palestinian security and medical sources said. An eighth guard was shot and killed by Israeli forces as he ran toward the border.
And three members of the Fatah security apparatus were killed in sporadic clashes in Gaza City, according to Palestinian medical and security sources.
Palestinian President Mahmoud Abbas is the leader of the rival Fatah party and the presidential guards are Fatah members.
Hours before Tuesday's attack on the border crossing, Ibrahim Mouniyeh, a member of Hamas' militant wing, Izzedine al Qassam, was killed outside his house, Palestinian security sources said
For months, Hamas and Fatah have been engaged in a power struggle, with their militias often battling in the streets of Gaza City. At least 18 people have died in the violence since Sunday.
An Egyptian-brokered cease-fire designed to end the fighting was first broken Monday and then again Tuesday.
The renewed violence came only hours after a Palestinian government spokesman said the cease-fire between Hamas and Fatah had been restored.
"We have an agreement, and we have confidence that this agreement should be implemented this night," Ghazi Hamad told reporters in Gaza City late Monday.
Abbas and Palestinian Prime Minister Ismail Haniya had issued orders to security forces from Fatah and Hamas respectively to withdraw from the streets, Hamad said.
"Hamas and Fatah -- both of them -- give orders to their elements in the street to withdraw and to stop all military conflicts and clashes in the streets and to return all kidnapped people and to stop the media inciting," he said.
The cease-fire was initially broken early Monday when shooting resumed between rival militias, leaving two gunmen dead and 10 people wounded, Palestinian security sources said.
Violence between the factions left four dead Sunday.
In protest of the ongoing violence in Gaza, Palestinian Interior Minister Hani al-Qawasmi resigned Monday, one of his assistants told CNN. According to the assistant, Haniya accepted al-Qawasmi's resignation.
His resignation was a blow to the Palestinian unity government, formed earlier this year in an effort to end the deadly factional fighting concentrated in Gaza.
One of the major sticking points in forming the Fatah-Hamas government has been the portfolio of the interior minister and choosing who would fill the key position.
The interior minister has control of Palestinian security forces.
Amid frustrations over the inability to enact a Gaza security plan, al-Qawasmi had tendered his resignation to Haniya a few weeks ago, but the prime minister did not accept it.
In the initial installment of the cease-fire, Hamas and Fatah officials told reporters shortly after midnight Monday that they would begin observing the truce at 1 a.m. (6 p.m. Sunday ET), but the cease-fire never made it to daybreak.
Under the agreement, all checkpoints were required to be dismantled, gunmen from both sides were required to be withdrawn from the streets and any hostages held by either side were to be released.
Last year, Hamas ended decades of Fatah rule by winning Palestinian elections.
After months of deadly clashes, the two sides met recently to build a new government aimed at ending the fighting.