Masked gunmen have blown up part of a United Nations club, in the latest sign of spiralling unrest in the Gaza Strip.The attackers tied up the security guard and beat him before setting off explosives in the drinking hall, one of the few places in conservative Muslim Gaza where alcohol is served. There was no immediate claim of responsibility. As the Palestinian territory’s second largest employer, the UN is generally viewed sympathetically. The bombing is another blow for President Mahmoud Abbas, just hours after militants announced the end of a de facto truce with Israel. Meanwhile, Israeli artillery fire has killed two Palestinian militants in Gaza after rocket attacks were launched from the area. The deaths were the first such casualties since Israel imposed a so-called “no go zone” in the north of territory to curb strikes by insurgents. Abbas vowed last night to re-impose order on Gaza, following the kidnapping of three Britons and a surge in internal unrest.