KABUL, Afghanistan - The death toll from an attack against the Kabul office of the Afghan president's running mate climbed to at least 20 people on Monday, an official said.

Around 50 other people were wounded in Sunday's attack against the Green Trend party office, which lasted hours and included a gunbattle between security forces and the attackers, who were holed up inside the building, according to Interior Ministry spokesman Nasrat Rahimi.

Several gunmen were killed by the security forces, Rahimi said.

The attackers' potential target, vice-presidential candidate and former intelligence chief Amrullah Saleh, was "evacuated from the building and moved to a safe location", Rahimi said. About 85 other civilians were also rescued from inside.

No one immediately claimed responsibility for the attack, but both the Taliban and the Islamic State group are active in the capital and have carried out large-scale attacks in Kabul in the past.

The Taliban, who effectively control half the country at this point, have also been staging near-daily attacks across Afghanistan even as they hold talks with the United States about a peaceful resolution to the 18-year war, Washington's longest conflict. The insurgents however, refuse to directly negotiate with the government, calling it "a US puppet".

Sunday marked the first day of the Afghan presidential campaign, with a vote scheduled for the end of September.

After the attack, Afghan President Ashraf Ghani tweeted that Saleh was unharmed during the "complex attack" targeting the Green Trend office.

Saleh founded the Green Trend party after he was sacked as intelligence chief in 2010 by former president Hamid Karzai. Though a relative newcomer on the Afghan political scene, the party's focus has been democracy and reform while fiercely opposing the Taliban and their extremist ideology.

Ferdous Faramarz, the spokesman for Kabul's police chief, said the attack started with a suicide car bombing, after which other attackers entered the building and started shooting at security forces.

The explosion from the initial bombing was large enough to be heard throughout the capital.

Ghani is seeking a second term in the Sept 28 vote on promises of ending the war but has been largely sidelined over the past year amid US-Taliban talks.

US envoy Zalmay Khalilzad, who is currently visiting Kabul, has held several rounds of talks with the Taliban in recent months. The two sides appear to be closing in on an agreement in which the US would withdraw its forces in return for a pledge from the Taliban to keep the country from being used as a launchpad for global attacks.

Locals slam the government over what they described as "security lapse," calling upon the security apparatus to protect the lives and properties of people during elections.

"Some of the victims even don't have money to rebuild their houses damaged by the blast on Sunday," Kabul resident Samiul Haq said.

Agencies - Xinhua