Arab nations have condemned the latest infighting in Gaza, pledging fresh support for the embattled Palestinian Authority President Mahmoud Abbas.
There are fears fighting may reach the Fatah-dominated West Bank
His Fatah faction has been ousted from Gaza by rival Hamas militants, ending a week of fierce feuding in which at least 100 people died.
After talks in Cairo, Arab League foreign ministers urged Hamas and Fatah to end their rivalry and co-operate.
A key Hamas leader earlier said he remained willing to work with Mr Abbas.
Khaled Meshaal, Hamas's political leader, who lives in exile in Syria, said Mr Abbas remained the "legitimate" president.
However, Hamas declared as illegal an earlier decree by Mr Abbas that replaced its Prime Minister, Ismail Haniya.
Mr Abbas, who is in the West Bank, named former Finance Minister Salam Fayyad as his new prime minister after dissolving the Hamas-led national unity government.
Following emergency talks between Arab League foreign ministers, the body's spokesman, Amr Moussa, said there must be an "immediate and full halt" to the violence, which he described as criminal.
"We are seeking a national unity in Palestine and we stand against the events that we have witnessed in the last days," he said.
He said Arab nations wanted to "serve the Palestinian cause, and not one faction against another".
The Arab League agreed to form a committee that would support efforts by Saudi Arabia and Egypt to mediate between Fatah and Hamas.
According to the BBC's Heba Saleh in Cairo, these Arab governments regard the Palestinian problem as a festering sore breeding militancy and radicalising populations in the region.
The group of Middle East mediators known as the Quartet - the US, UN, EU and Russia - have also thrown their weight behind Mr Abbas, pledging him their "full support".
A US state department spokesman said the Quartet was examining ways of helping a new Palestinian government, headed by Mr Abbas's nominee.
"This is a government that intends to abide by the principles that previous Palestinian governments had prior to the Hamas-led government," the spokesman, Sean McCormack, said.
President Abbas dismissed the three-month-old unity government on Thursday and declared a state of emergency.
Born in 1952 near West Bank city of Tulkarm
Holds a PhD in economics from the University of Texas
Worked at the World Bank in Washington from 1987-1995
IMF representative to Palestine until 2001
Finance minister under the Fatah-controlled administration from 2002-2005
Credited with cracking down on official corruption
He said he would rule by presidential decree until the conditions were right for early elections.
His nominee to replace Mr Haniya, Mr Fayyad, is a former World Bank economist.
Western nations have boycotted Hamas, considered a terrorist organisation by the US and EU, since it won elections in January 2006.
The BBC's Matthew Price in Jerusalem says the West Bank and Gaza Strip will now effectively be split from one another - Gaza run by the Islamist Hamas and the West Bank by the secular Fatah.
There are also fears that violence will spread to the West Bank, where Fatah is dominant.
Human Rights Watch has urged both factions to treat civilians humanely and prevent summary killings.
Joe Stork, deputy Middle East director, said: "Killing people who are in your custody and under your control... constitutes an extremely grave offence, in fact, a war crime."