TRIPOLI, May 3 (Xinhua) -- The death toll in an attack by Islamic State (IS) suicide bombers on the headquarters of the Libyan Higher Commission of Elections here Wednesday has risen to 15, according to a medical source.

Confirming the deaths, Abdaddaem al-Rabti, an official of the public Tripoli Field Hospital, told Xinhua late Wednesday that nine commission employees, four security personnel and two terrorists had died while 21 people were injured.

The terrorists stormed the building's main gate on Wednesday, exchanged fire with the security, and blew themselves up after being surrounded by backup security personnel.

The attack took place as the UN-backed government, in cooperation with the UN Support Mission in Libya, prepares to hold presidential and parliamentary elections before the end of this year, as proposed by the head of the mission, Ghassan Salame.

Abdalhakim Belkhair, deputy head of the commission, told Xinhua that the attack "seeks to send out clear messages attempting to thwart the upcoming elections in any way".

The UN-backed Libyan government declared three days' national mourning for the victims, saying, "These crimes will only increase our determination to unite against them and to fight terrorism in all forms."

Libya has remained mired in chaos following the 2011 uprising that toppled former leader Muammar Gaddafi. It is struggling to make a democratic transition amid political division and unrest, as well as dominance of armed groups and militias with shifting loyalties.