* Four suicide bombers killed by Afghan forces
* Eight UN staff inside besieged building
* Afghan capital on high alert after wave of attacks
* Second assault on election body in Kabul (Updates with end of attack, militants killed)
By Mirwais Harooni and Jessica Donati
KABUL, March 29 (Reuters) - Taliban insurgents attacked the Independent Election Commission headquarters in Kabul on Saturday, staff and police said, in the their third big assault on the capital this week aimed at derailing the April 5 presidential election.
Afghan security forces battled the militants for about five hours, while frightened IEC staff and eight international United Nations employees took refuge in safe rooms inside the compound, a security source and staff said.
Four suicide bombers were involved in the attack and all were killed in gunbattles, according to an Afghan army general on the scene in the eastern part of the capital.
"The fight is over. All four terrorists have been killed. An investigation team is in the area," said commander Qadam Shah Shaheem, adding that three security force members had been injured in the operation.
No other casualties were reported.
With a week to go before Afghanistan's presidential election, escalating violence across the country risks undermining the credibility of a vote meant to mark the first democratic transfer of power in Afghan history.
It was the second IEC building this week to have been targeted by the radical Islamist Taliban. The attack occurred less than 24 hours after militants stormed a guesthouse used by a U.S.-based aid group. A child was killed in Friday's attack.
Kabul is on high alert ahead of the presidential election that Taliban insurgents have threatened to scuttle with a campaign of bombings and assassinations.
Election commission staff heard an initial explosion at around midday on Saturday at the IEC headquarters, followed by gunfire and rockets, one of which damaged a warehouse inside the compound, according to an IEC employee.
"Four suicide bombers armed with light and heavy weapons have entered a building near the IEC headquarters and are shooting towards the IEC compound and at passersby," Mohammad Zahir, the Kabul police chief, told reporters near the site of the attack at the time it was going on.
The IEC compound is close to the United Nations Office Complex in Afghanistan (UNOCA) and other international organisations.
"I am here ... The attack is going on around the IEC compound," commission spokesman Noor Mohammad Noor told Reuters by telephone from a safe-room inside the building.
He said that IEC personnel were safe and Afghan security forces were in control of their building.
U.N. staff at the complex near the IEC premises were instructed to take refuge in safe-rooms until further notice.
Earlier, the owner of a house used to shoot at the IEC building said that three guards were present at the time it was taken by Taliban militants who had disguised themselves as women in the traditional Islamic, all-covering burqa.
"I had three guards, two outside and one inside, but I don't know what is happening right now. The attackers were wearing women's burqas," said Haji Mohibullah.
Mohibullah's house was being used as an office by campaigners for presidential candidate Gul Agha Sherzai, according to the Afghan military.
Deputy interior minister Mohammad Ayub Salangi said later that five attackers had been involved and two Afghan security force members wounded in the battle.
"Nobody was killed or wounded in IEC headquarters... I visited personally the IEC headquarters and all was well."
TENSIONS RISE IN KABUL
Afghans are voting for a successor to President Hamid Karzai, who is constitutionally barred from running for another term in office.
The vote is seen as a major test by foreign donors who are hesitant about bankrolling the government after the bulk of NATO troops stationed in Afghanistan withdraw later this year.
The spate of attacks has rattled nerves in Kabul, and on Saturday, according to an internal United Nations note sent to staff, the fighting forced the airport to cancel all flights.
On Friday, Afghan security forces battled with the militants for hours before taking control of the guesthouse. The aid workers all survived, some by hiding in their rooms.
Insurgents have launched a campaign of so-called "complex" attacks in the capital, involving an initial bombing followed by gunfire. Dozens have also been killed in other assaults around the country in recent weeks.
Last week, nine people including an AFP journalist and an election observer were killed in an attack on a highly fortified hotel in the capital.
The hotel attack prompted most foreign election monitoring missions to withdraw their international observers, further diminishing confidence in the outcome of the vote next Saturday.
Taliban suicide bombers and gunmen assaulted an election commission office in Kabul on Tuesday. Ten people were killed in the attack, including a provincial council candidate. (Editing by Mark Heinrich)