Afghanistan's electoral commission says it has delayed voter registration in some parts of the south over security concerns.

Voter registration for the upcoming presidential election started in southern Afghanistan this week, but the commission says voters will not be able to register in parts of Helmand province until security improves.

Violence has surged in the south over the past year as Taliban fighters have intensified their attacks on Afghan and international troops.

Meanwhile, Afghan and international forces say they killed at least 28 militants late Wednesday and early Thursday in separate battles in the eastern province of Khost and the southern Zabul region.

NATO says its forces killed 22 militants in air strikes and ground fighting in Khost, near the border with Pakistan. Earlier, the Afghan defense ministry said Afghan troops in the same province killed eight militants and wounded two others. It is unclear whether they were referring to the same incident.

Meanwhile, the U.S. military says coalition forces killed six militants Wednesday during a raid on Taliban fighters blamed for roadside bomb attacks in Zabul.

The United States plans to boost its fight against the Taliban this year by sending an extra 30,000 troops to Afghanistan.

Some information for this report was provided by AFP, AP and Reuters.