JALALABAD - A suicide bomber killed three Turkish engineers and wounded another one on Monday in eastern Afghanistan, the latest attack on foreigners in the war-torn country.

It came after the US and the Taliban sealed a dramatic prisoner swap that saw soldier Bowe Bergdahl released for five senior insurgent figures, raising hopes for peace as foreign forces prepare to withdraw.

"Around 7:15am, a suicide bomber detonated his explosive-packed motorbike targeting a minibus belonging to Turkish engineers in Behsud district of Nangarhar province," Ahmad Zia Abdulzai, a spokesman for the provincial governor, told AFP. "As a result of this attack, three Turkish engineers were killed and the fourth one was wounded." Abdulzai said the victims were working on a construction project in Nangarhar and were travelling to work when their minibus was targeted.

Hazrat Hussain Mashriqiwal, spokesman for the Nangarhar police chief, said an Afghan child was also injured.

Turkey has 459 soldiers in the NATO-led forces in Afghanistan. The Turkish foreign ministry denounced what it called "a cowardly terrorist act" and called for those responsible quickly to be brought to justice.

There was no claim of responsibility for the attack. The Taliban, who announced their spring offensive in early May, were not immediately reachable for comment.

The attack comes as Afghanistan is in the middle of presidential elections, with former foreign minister Abdullah Abdullah and ex-World Bank economist Ashraf Ghani due to compete in a head-to-head run-off vote on June 14.

The Taliban Monday threatened to attack voters and officials taking part in the second round.

The militants had threatened to attack the first round of voting on April 5, but the day passed off with a high turnout and no major security incidents.

Both candidates have promised to bring peace after decades of conflict. But they will have to tackle a challenging security situation without NATO combat troops, all 51,000 of whom will pull out by the end of this year.

President Barack Obama last week outlined the US strategy to end America's longest war, saying that the 32,000-strong US deployment in Afghanistan would be scaled back to around 9,800 by the start of 2015.

Those forces would be halved by the end of 2015 before eventually being reduced to a normal embassy presence with a security assistance component by the end of 2016.

The US troops will stay only if a key security pact, the Bilateral Security Agreement, is signed between Kabul and Washington. The outgoing president Hamid Karzai refuses to sign the pact, but both presidential candidates have vowed to do so if elected.