The attack comes as aid agencies in neighbouring Pakistan suspended operations over fears of an attack by al-Qaida.
The four-vehicle convoy was carrying about 15 international and local staff, who are working on Afghanistan's upcoming elections, when it passed by two landmines in southeastern Paktia province, Manoel de Almeida e Silva said on Sunday.
Neither mine blast hit any of the vehicles but the cars were then attacked by an unknown number of assailants who used "small arms and possibly RPGs (rocket-propelled grenades)," the spokesman said.
Afghan police who were providing security for the convoy engaged the attackers and the US-led occupation force was called in to provide assistance, de Almeida e Silva said, without giving further details.
"Initial reports are that no one was hurt in the convoy and the convoy continued through to Gardez," he said.
The vehicles were travelling between the Pakistan border province of Khost and Paktia's capital Gardez when the attack occurred in the Schwak district on Sunday morning.
Remnants of the ousted Taliban regime have threatened to disrupt Afghanistan's landmark September elections, which are being organised with assistance from the UN.
US-led forces proxies are hunting
Taliban and al-Qaida forces
On Wednesday, three European and two Afghan aid workers with Medecins Sans Frontieres (Doctors Without Borders), were killed by suspected Taliban as they were travelling in northwestern Baghdis province.
In recent weeks there has been a surge in attacks by anti-government forces, particularly in the south and southeast where the 20,000-strong US-led force is deployed in strength to hunt and kill Taliban, al-Qaida and other forces.
Aid workers withdraw
Meanwhile, across the border, several foreign aid groups suspended their operations in southwestern Pakistan and moved staff out of the region after threats of attacks from an al-Qaida-linked fighter, officials said.
"We have suspended our operation and shifted about a dozen foreign staff members to hotels or other safer places," said United Nations refugee agency (UNHCR) spokesman Babar Baluch.
Police said around 18 other foreigners, including US and British nationals, working with various non-government organisations (NGOs) had also moved out of the region, which borders Afghanistan.
Intelligence officials had received information that a former Taliban fighter - Maulvi Hashim Ishaqzai - was planning attacks on UNHCR offices in Quetta, the capital of southwestern Baluchistan province, and the border town of Chaman, provincial police chief Chaudhry Yaqub said.