"The bomb went off, we believe, outside the headquarters of the traffic police ... and close to the main bus stop that people queue at in order to take the bus from Helmand to Kandahar," he said.

'Taliban revenge'

Gulab Mangal, the governor of Helmand, has already blamed the Taliban for the attack.

"The Taliban have almost been defeated in Marjah, now they are taking their revenge big time on the people of Lashkar Gah," he told Al Jazeera, referring to the area where Nato and Afghan forces are carrying out a joint offensive against the Taliban.

In depth

  Operation Moshtarak at a glance
  Gallery: Operation Moshtarak
  Video: Taliban fighter says Nato losing Afghan support
  Video: Civilians flee Marjah fighting
  Video: Pakistan's motives questioned
  Focus: To win over Afghans, US must listen
  Timeline: Afghanistan in crisis

Another blast in the northern city of Jalalabad, meanwhile, killed one person and injured five more on Tuesday.

In other developments, the US media reported on Tuesday that yet another senior Afghan Taliban commander had been captured in Pakistan.

Mullah Abdul Kabir, a member of the group's so-called Quetta Shura, was picked up several days ago in Nowshera district in Pakistan's northwest, according to reports in the New York Times and Washington Post that cited unnamed Pakistani security officials.

Pakistani officials declined to confirm the reports.

Last week, Pakistani and American officials said that Mullah Abdul Ghani Brader, the No 2 Afghan Taliban leader, had been caught 10 days earlier in the southern Pakistani city of Karachi.

Two other Taliban leaders - Mullah Abdul Salam and Mullah Mohammad, respectively the "shadow governors" of Kunduz and Baghlan provinces - were arrested separately in Pakistan about 10 to 12 days ago, according to Mohammad Omar, the Kunduz governor.