Afghanistan's parliamentary elections are scheduled for September 18, and security is a major concern.
Last year's presidential elections were marred by dozens of attacks on voters and polling places.
Khost is one of Afghanistan's least secure provinces. Anti-government groups staged 364 attacks there between April and June, up from 240 during the same period in 2009, according to a recent report from the Afghanistan NGO Security Office.
The Haqqani network, one of Afghanistan's three main insurgent groups, is particularly active in Khost, which shares a border with Pakistan.
Meanwhile, the Nato-led International Security Assistance Force (Isaf) said it detained several "Taliban figures" in overnight operations, including the group's former spokesman.
Abdul Hay Motmaen, a spokesman for the Taliban when it ruled Afghanistan, was arrested in Ghazni province on Thursday night, Isaf said in a statement.
Two other suspected mid-level insurgents were arrested in Kandahar province earlier this week.