About 500 Fatah loyalists have returned to Gaza from police training in Egypt amid fighting with rival faction Hamas.
The latest factional fighting is the worst in several months
The latest violence killed 14 people, including eight in one attack on Fatah - the worst in three days of bloodshed.
A Fatah official denied the loyalists would fight Hamas, saying they give Palestinians security.
Four Israelis were injured, with one woman seriously hurt, in the town of Sderot, near Gaza, by a rocket fired by Palestinian militants.
Palestinian Authority President Mahmoud Abbas has urged an end to the factional fighting.
There were reports on Tuesday night that the rival groups had agreed a ceasefire deal. However, several similar accords have broken down within hours.
The BBC's Katya Adler in Jerusalem says the latest events in Gaza appear to be a significant escalation in the fighting, pushing the area closer to all-out civil conflict.
The Fatah official, Tawfiq Abu Khoussa, said of the returning men: "The role of the security forces is to protect the security of the Palestinian people and not to take part in internal fighting."
The force is reported to be under the command of Muhammad Dahlan, national security adviser to Mr Abbas, who also leads Fatah.
At least 20 people have died in Gaza since Sunday in the worst outbreak of factional violence in several months.
Gun battles continued to rage on the streets of Gaza City on Tuesday. Schools and businesses were closed and most residents kept indoors.
Tuesday morning's attack on the Fatah base took place near the key Karni crossing point, and drew fire from Israeli troops guarding the border.
Reports say the fighting erupted when suspected Hamas gunmen approached a training base used by the pro-Fatah Presidential Guard which is responsible for security on the Palestinian side of the Karni crossing.
A Presidential Guard spokesman told Associated Press the Karni base was attacked with rockets, rocket-propelled grenades and mortars.
After the ambush, bodies were seen strewn in the grass near an overturned security vehicle.
A spokesman for the Hamas paramilitary Executive Force denied its members were involved in the clash.
The head of the Palestinian Energy Authority said electricity in Gaza could be shut down by Wednesday because the fighting near Karni had stopped fuel trucks arriving.
Also on Tuesday, Hamas accused Fatah of killing one of its commanders, and Fatah said Hamas killed a pro-Fatah security officer and wounded three others near Gaza City.
The fighting, and Monday's resignation of Interior Minister Hani Qawasmi, have dealt a major blow to the two-month-old national unity government set up by Islamist Hamas and its secular rival Fatah.
An Israeli tank observes the attack on the base near Karni
In a speech marking Nakba (Catastrophe) Day, the anniversary of the establishment of Israel in 1948, Mr Abbas called for an immediate implementation of a security plan to halt the internal fighting.
In a separate speech, Prime Minister Ismail Haniya of Hamas urged people to work together to protect the national unity government.
UK Foreign Secretary Margaret Beckett said she was "deeply concerned" by the rising violence.
She said: "This fighting is senseless: it endangers civilians, makes it harder for the international community to help the people of Gaza and undermines the peace process."
Up to 170 people have died in clashes between Fatah and Hamas since the latter won parliamentary elections in January 2006.
Israel withdrew its settlers and troops from Gaza in 2005, but kept control of its borders, airspace and territorial water.