In a makeshift boot camp, dozens of Palestinians charged across a field on Wednesday, assault rifles aimed at an invisible enemy.
But for the gunmen from the Fatah Party, the target was clear: the private army recently formed by the Hamas-led government.
"We are going to confront this Hamas unit," said one 22-year-old Fatah fighter who only gave his nom de guerre, Abu Satter.
Just a few kilometers from the Fatah camp in southern Gaza, Hamas fighters drilled on how to whisk suspects from a car, cuff and guard them. In another sign of gearing for possible battle, Hamas recently bought a black market shipment of 100,000 bullets after outbidding Fatah, according to one official involved in the negotiations.
Both sides insist they will not be drawn into civil war.
However, tensions have risen sharply since Hamas took power in March, after its sweeping victory in January parliament elections.
In just four weeks in office, Hamas has been backed into a corner.
It can't pay its 165,000 employees, the backbone of the labor force, because the West has frozen aid and blocked money transfers from the Arab world. Shops and streets are empty as Palestinians scrape by on savings.
At the same time, President Mahmoud Abbas, who wields considerable power vis-a-vis Hamas, has stripped the government of powers in recent weeks, assuming control of the border authority and the state-run TV and radio.
Initially, the tone between the two sides was civil. Hamas tried not to alienate Abbas, seen as its only shot at international respectability. Abbas, as leader of Fatah, would lose credibility at home if he was perceived as trying to bring down the government of a rival party.
However, wrangling over control of the security forces has laid bare the deep enmity.
Abbas holds the title of general commander of the security force, which consists of five branches. Three are under the control of the Hamas-run Interior Ministry, while two report directly to Abbas.
Last month Abbas appointed Rashid Abu Shbak, as commander of one of the Hamas-controlled branches, the powerful Preventive Security Service. In the 1990s, Abu Shbak had led a crackdown on Hamas, and his appointment was seen by Hamas as a provocation.
Hamas responded by announcing the creation of a special unit, to consist of some 4,000 militants. Abbas annulled the decision, but Hamas refused to back down, and the standoff continues.