* Police official says 7 foreign workers killed
* Security sources say 6 dead, including 3 foreigners
(Adds foreigners killed in attack on contractor)
By Ismail Sameem
KANDAHAR, Afghanistan, April 15 (Reuters) - A suicide car bomber struck the compound of a foreign security firm in the southern Afghan city of Kandahar on Thursday, killing several foreigners and Afghan guards, officials said.
The blast came hours after another large car bomb in the centre of the city, which has been the scene of numerous attacks in recent weeks ahead of a major planned offensive by U.S. troops expected there in coming months.
"The attack was on a security company. There are some casualties. Some foreigners were injured and killed; some security guards were killed," Ahmad Wali Karzai, head of the Kandahar provincial council, told Reuters.
He said preliminary reports indicated at least three foreigners had been killed and nine wounded.
A police official, Mohammad Nabi, said seven foreign workers, believed to be British, had been killed in the blast. Two other security sources said there were six dead, including three foreigners.
A doctor at a central Kandahar hospital, Farhad, who uses only one name, said one dead foreigner had been brought to its morgue. Sixteen wounded Afghans and one wounded foreigner had been treated there, he said.
The attack came hours after another car bomb exploded near a hotel, destroying shops and cars. Nabi said six people were wounded in that first attack.
The birthplace of the Taliban, Kandahar is to be the focus of an upcoming U.S.-led military offensive to turn the tide against a stubborn insurgency, and the militant group in the past has played up some attacks as a warning to NATO forces.
Earlier this week, a group of three suicide bombers tried to seize the main intelligence building in Kandahar in a brazen daylight raid.
In the biggest attack in Kandahar this year, Taliban suicide bombers killed more than 30 people in a series of coordinated strikes across the city last month.
The bulk of the 30,000 additional combat troops ordered to Afghanistan by U.S. President Barack Obama last year are expected to be deployed to Kandahar as part of the summer campaign to root out the Taliban from their spiritual homeland.
Residents and security officials say the insurgent group has been waging a campaign of intimidation and attacks in an effort to drum up local support forcibly and hamper operations aimed at winning the trust of ordinary Afghans.