Seven civilians were killed Monday in a blast in the southern Afghan province of Helmand, a spokesman for provincial government said on Tuesday.

"Seven civilians were killed when a powerful blast occurred at around 4:00 p.m.local time Monday in a compound in Naw Zad district of Helmand province," spokesman Daud Ahmadi told Xinhua.

He said four women and three children were killed in the explosion that happened in a house in Sarkhdaz area of the district.

Ahmadi said it was not clear how the explosion happened, adding an investigation was launched to probe why the explosive was placed in the house.

Taliban militants have often attacked Afghan and NATO-led forces with homemade bombs or Improvised Explosive Devices (IEDs) but the lethal weapons also inflict casualties on civilians.

The number of civilian casualties has been soaring in the militancy-plagued Afghanistan as a total of 3,021 Afghan civilians have been killed in 2011 which indicates, an 8 percent rise in non- combatants' deaths compared with 2010, according to the United Nations annual report released in Kabul earlier this month.

Notorious for growing poppy and militancy, the Helmand province has been experiencing increasing militancy over the past couples of years despite continued military operations launched by Afghan forces and NATO-led troops.