A pair of suicide bombers attacked an army camp in northwestern Pakistan Wednesday, killing at least three soldiers and wounding more than 20 others.

Security officials say the attackers rammed their vehicle packed with explosives into a gate at the camp in Wana, the main town in the South Waziristan tribal area. There was no immediate claim of responsibility.

South Waziristan is one of Pakistan's seven federally administered tribal agencies. The military has launched several offensives in the region targeting the Taliban and militants linked to al-Qaida who have strongholds in the porous border area.

U.S. drone strikes, which are unpopular in Pakistan, also regularly target militants in the region.

Last week, a suicide bombing in Wana wounded Maulvi Nazir, the main militant commander in South Waziristan.

Separately, the “Global Index on Terrorism 2012” report released Tuesday ranked Pakistan the nation second most affected by terrorism in 2011, out of 158 countries.

The report put out by the Australia-based Institute for Economics and Peace says terrorist attacks in Pakistan last year killed nearly 1,500 people.

Thousands of Pakistanis have been killed in terrorist attacks since the country joined the U.S.-led war on terror in the region following the attacks on New York and Washington in 2001.

Recently, Pakistan has faced a fresh wave of sectarian violence, mainly in southern Baluchistan province.