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Palestinian gunmen from the Fatah movement fire toward a Hamas fighter during an exchange of fire Wednesday at a demonstration in Gaza City. Gunfire killed one of the demonstrators and wounded several others. ((Khalil Hamra/Associated Press))

Hamas forces appeared to ignore international calls for a ceasefire Wednesday, continuingto bombard andoverrun rival Fatah's security positions in the Gaza Strip as factional violence spread for a fourth day.

More than15 people were killed in Wednesday'srenewed fighting between the secular Fatah movement, led by Palestinian Authority President Mahmoud Abbas, and the Islamist Hamas, whose political branch won the mostseats in the January 2006 parliamentary election.Each side has accused the other of escalating the violence.

Hamas fighters also gained control of a major road on the coastal strip, potentially cutting off overwhelmed Fatah forces from reinforcementsas part ofan Hamas offensive with the apparent aim of gaining total of Gaza.

The violence spread to central Gaza and key strategic positions within Gaza City as Hamas fought for control of highrise buildings that serve as sniper positions.

In one battle, hundreds of Fatah-allied fighters surrendered to masked Hamasgunmen after an intense battle. They were then led, arms in air, to a nearby mosque.

At least sixpeople were killed in aseparatebattlenear the besieged house of a senior Fatah commander in Gaza City.

Abbas warns of 'collapse'

From the West Bank, Abbas denounced the Gaza fighting as "madness" and warned of an imminent collapse in the region. He again appealed to Hamas's exiled leader, Khaled Mashaal, to call an end to the attacks.

Gunmen linked to both factions have been battling for months even though the leaders agreed in February to form a coalition government — admittedly, one that has since been paralyzed by in-fighting over who controls key parts of the government, including security forces.

The Hamas government was forced to form the coalition amid a Western-led boycott (over Hamas's refusal to renounce violence and recognize Israel) that shut off billions of dollars in aid to the Palestinians and amid the battles between Hamas and Fatah supporters.

An Egyptian-mediated truce agreed to by both factions late Monday was shattered almost instantly. The United Nations, the European Union, Russia and the Arab League have called for an immediate ceasefire.

The U.S. State Department denounced the latest violence as a direct attack by the most radical elements of Hamas on legitimate Palestinian authorities. Spokesman Sean McCormack said Washington had no indication that Israel might intervene to try to stop the infighting.

But Hamas leaders blamed the intensified fighting on Abbas, accusing his security forces of corruption and being riddled with criminals.

Palestinian unity government spokesman Mustafa Barghouti told CBC News Wednesday that the world community was also to blame for the increased bloodshed because of what he said was a refusal to empower the fragile coalition.

"What could collapse is not just the national unity government, but the whole [Palestinian] authority, and it will be replaced by chaos, lawlessness and a terrible situation," Barghouti said in a telephone from Belgium while en route to Gaza.

Barghouti described the current situation as one of "severe lawlessness," but denied the fighting qualified as outright civil war.

Anti-violence protest draws gunfire

Also Wednesday,a demonstration ofabout 1,000 demonstrators calling for a halt to theviolence was greeted by gunfire that killed a 16-year-old protester and wounded several others.

Two other people died Wednesday from wounds sustained in earlier fighting.

The attacks come a day after about 200 Hamas fighters seized control of Fatah's main security compound following heavy shelling and several hours of fighting. At least 17 were killed in the battle, accordingto officialsfrom both sides.

With files from the Associated Press