KABUL, Afghanistan - Afghan security forces have arrested five extremists with 22,000 pounds of explosives that they smuggled in from Pakistan to carry out a huge attack in Kabul, as well as another three suspects allegedly planning to assassinate the vice president, an official said Saturday.

The reports of new planned attacks in the Afghan capital came a week after extremists said to be part of the Pakistan-based Haqqani group launched coordinated assaults in the heart of Kabul and in three other cities.

U.S. officials say they have stepped up pressure on Islamabad to crack down on the Haqqanis, who specialize in high-profile strikes against well-protected targets.

Three of the five men arrested with the explosives were members of the Pakistani Taliban, while the two others belonged to the Afghan Taliban, National Director for Security spokesman Shafiqullah Tahiry told reporters. He said the men's orders came from extremist leaders with ties to Pakistani intelligence. He did not say when the arrests took place, nor what their intended target was.

Tahiry said the seized explosives were packed in 400 bags and hidden under potatoes loaded in a truck with Pakistani license plates.

The men confessed that they "had planned to carry out a terrorist attack in a key point in Kabul city," Tahiry said.

He provided a DVD showing images of the truck and the recorded confessions of the men, but he did not provide other proof to back up the claims.

He said that the three Pakistani members of the group picked up the explosives just outside the Pakistani city of Peshawar, and were under the orders of two local Taliban leaders named Noor Afzal and Mohammad Omar, who Tahiry said had ties to the country's Inter-Services Intelligence agency.

Tahiry also said that security forces had foiled an assassination attempt by the Haqqani network against Afghan Vice President Mohammed Karim Khalili.

He said that three Afghan men arrested April 15, the day the Kabul attacks began, planned to kill Khalili at his home. They were equipped with suicide vests and small arms.

According to Tahiry, the order to kill Khalili was issued in Miram Shah by Haqqani network commander Badruddin Haqqani, the son of the group's founder, Jalaluddin Haqqani. Last May, the United States designated Badruddin a terrorist.