KABUL: Security forces in the Afghan capital have defused a truck bomb packed with nearly eight tonnes of explosives, the biggest of its kind discovered in the country, the spy agency said on Friday amid heightened security.

Intelligence forces discovered the explosives in eastern Kabul, wired and ready for detonation, security officials said. Five al-Qaeda linked Haqqani network insurgents were killed in a resulting firefight.

"This truck bomb could have destroyed an area around 1.5 km (in radius). Now can you imagine that what kind of catastrophe this would be?," Shafiqullah Tahiri, a spokesman for the National Directorate of Security, told a news conference.

It contained sodium chloride, ammonium nitrate, other chemicals and some diesel.

After more than 11 years of war, insurgents are still able to strike strategic military targets and launch high-profile attacks in Kabul and elsewhere.

The truck bomb was discovered on Wednesday two days after U.S. Defense Secretary Chuck Hagel left the capital. On the first full day of Hagel's visit, a suicide bomber struck about a kilometre away from his morning meetings at a NATO facility.

Two Haqqani operatives were arrested during the night raid, and Tahiri said the militants had been planning to target a military facility in the capital. There are several foreign and Afghan military bases in Kabul, housing thousands of soldiers.

The Haqqani network is widely regarded as the most dangerous U.S. foe in Afghanistan.

Afghanistan is bracing for the start of the spring fighting season, and officials fear that militants will infiltrate the capital as the snow melts in the mountains to the east where they hide.

A number of insurgent groups, including the Taliban, the Haqqani network and Hizb-i-Islami, led by Gulbuddin Hekmatyar, are known to reside in restive Wardak province just 40 minutes drive from Kabul.

Afghan officials told Reuters this week they were worried that the Afghan capital would be in danger after President Hamid Karzai ordered U.S. special forces tasked with fighting the Taliban to leave Wardak.

U.S. special forces are expected to play a major role after most NATO combat troops withdraw by the end of 2014, and Karzai's decision could complicate negotiations between the United States and Afghanistan over the scope of U.S. operations after the pull-out.

International forces in Afghanistan have also been warned the that recent inflammatory remarks by Karzai have put them at risk, NATO commander General Joseph Dunford said in an email obtained by the New York Times this week.

Karzai accused the United States of colluding with the Taliban hours before the two met last Sunday. His remarks, which have been rejected by Washington, highlight an often tense relationship with the United States.