On Friday night, a combined international and Afghan commando force captured a Taliban bomb-making expert on Friday night in the southwestern district of Wasel Abad of the capital Kabul, Nato said.
Neither Afghan nor Nato officials would identify the suspect, give details of the plot, or say how advanced the planning was.
But the Afghan defence ministry said several "enemies of the people" were killed in the raid and 26 people were arrested.
The alleged plot comes amid an escalating death toll in recent weeks as Afghan and US-led forces step up patrols in the Taliban-dominated south in a push to wrest control of the group's traditional stronghold.
Highlighting the increasing violence, five soldiers were killed in roadside bombings in Afghanistan on Friday and Saturday.
An US soldier in the east and a British soldier in the south were killed by homemade bombs on Saturday, while an explosion on Friday in the south claimed a British marine and another US soldier, Nato and Britain's defence ministry said.
Nato did not disclose the nationality or any details of the fifth soldier, who died in the south on Saturday.
In Kandahar, an Afghan policeman was shot and killed on Saturday, said Mohammad Shah Farooqi, head of the crime and investigation department of the province's police.
Last month was the deadliest of the nearly nine-year-old war for international troops, with 103 killed, including 60 Americans.
So far in July 54 have died, 39 of them from the US.
|Tuesday's conference will reportedly detail a 2014 security handover to Afghan forces [AFP]
The countries fighting in Afghanistan will agree at Tuesday's conference to hand over control of security to Afghan forces by 2014, reported British newspaper the Independent on Sunday.
A leaked communique circulated ahead of Tuesday's meeting of donor nations in Kabul will set out a phased transition beginning this year, the paper said.
"The international community expressed its support for the president of Afghanistan's objective that the Afghan National Security Forces (ANSF) should lead and conduct military operations in all provinces by the end of 2014," the agreed statement says, according to the paper.
It also pledges that foreign powers will continue to "provide the support necessary to increase security during this time, and the continued support in training, equipping and providing interim financing to the ANSF".