KANDAHAR, Afghanistan - At least 27 people, most of them civilians and one a provincial mayor, were killed in a spate of attacks in Afghanistan on Monday, officials said.

Violence has sharply increased in recent years in Afghanistan despite the growing number of NATO and U.S. troops, more than seven years on since the Islamist Taliban were ousted from power by U.S.-backed Afghan forces.

In the bloodiest incident on Monday, 12 civilians -- four women, two children and six men -- were killed by a roadside bomb that struck as they drove in a tractor in the Shamolzai district of southern Zabul province, said Mohammad Wazir, district chief of Shamolzai.

This was a mine newly planted by the Taliban, he told Reuters.

A while later, Taliban guerrillas ambushed a convoy of a security firm in another area of Zabul, killing six Afghan security guards in the convoy and two civilians nearby, Ghulam Jailani, a senior provincial police official, said.

Earlier on Monday, a provincial mayor was among seven people killed by a teenage suicide bomber who blew himself up at the gate of a municipal administration building in the eastern province of Laghman, the Interior Ministry said.

Three body guards and three civilians were killed along with the province's mayor, Mohammad Rahim, the Interior Ministry said.

A spokesman for the provincial governor's office, Sayed Ahmad Sopai, said 10 people were also wounded, including three women. He said the suicide bomber was identified as a 14-year-old boy from Paktika province further south.

There was no immediate claim of responsibility for any of the attacks. The Taliban have frequently used suicide bombers to strike government buildings as part of their campaign to drive foreign forces from Afghanistan.

The militant group has extended the size and scope of its activity and has carried out a number of high-profile attacks in major cities since last year.

To fight the Taliban insurgency, the United States plans to more than double its forces in Afghanistan this year, from 32,000 at the start of the year to a projected 68,000 by year's end. Other Western countries have about 30,000 troops in Afghanistan.

(Additional reporting by Sayed Salahuddin and Rafiq Sherzad in Kabul; writing by Sayed Salahuddin; editing by Peter Graff and Sugita Katyal)