GAZA, June 14 (RIA Novosti) - Fighters of Islamist group Hamas made key gains in furious gunbattles gripping Gaza, seizing the main security stronghold of rival pro-presidential group Fatah in Gaza City.

For the first time since the latest surge of violence began five days ago, Palestinian President Mahmoud Abbas ordered the presidential guard to strike back at Hamas.

A security source told RIA Novosti earlier that Fatah forces had managed to repel several attempts to storm the Preventative Security headquarters, and had also taken control of high-rise buildings nearby. However, after several hours of fighting in southwest Gaza City, Hamas fighters took control of the complex and hoisted their green flag on top of the headquarters.

The seizure of the heavily-guarded building signals a key advance for Hamas, who have had a score to settle with the Preventative Security Service since its mass arrests of Hamas members in the late 1990s.

More than 70 people have died since the fighting escalated on Saturday, despite a so-called truce being reached between the factions on Monday. Residents of Gaza City have been stranded in their homes amid continual gunfire and explosions, and hospitals are struggling to cope with large numbers of wounded, with limited resources and no electricity.

Hamas is still battling Fatah forces for control of key political and security command centers in Gaza City.

Israel has pledged not to intervene in the conflict unless Hamas, which it considers a terrorist organization, assumes full control over Gaza and threatens Israel.

The Israeli Armed Forces said all checkpoints on the Gaza border have been closed, including a terminal used to provide much-needed humanitarian aid to the impoverished, over-crowded Gaza Strip.

Mahmoud Abbas, who is widely supported by the West and whose presidential guard is funded by the United States, is expected to make a key announcement on the government's future later in the day. The leader is widely expected to withdraw Fatah from the governing coalition with Hamas, which it joined in March. Fatah threatened to pull out of the coalition in an announcement on Tuesday.