Kabul, Jan 12 (Agencies): The Taliban took aim at Afghanistan's intelligence services on Wednesday, killing six people and wounding more than 30 in two separate attacks, including a suicide bombing on a bus in the capital, officials said.

The attacks follow a surprise visit to Kabul a day earlier by US Vice President Joe Biden, who praised advances made against the insurgency while noting that the gains were "fragile and reversible." Biden left Afghanistan for neighboring Pakistan this morning.

In Kabul, a suicide bomber on a motorbike blew himself up next to a minibus carrying intelligence service employees to work, killing four people, President Hamid Karzai's office said.

The Interior Ministry said the blast wounded 32 people, including six intelligence service members _ although it spoke of only two people killed _ one intelligence service member and a civilian. The reason for the discrepancy was not immediately clear.

About an hour later in the troubled eastern province of Kunar, a remote-controlled roadside bomb killed an intelligence service colonel and his driver, and wounded two bodyguards, said Abdul Saboor Allahyar, deputy chief of Kunar's provincial police.

Insurgents often target Afghan security officials, although over the past few months the nation's capital has been largely spared the worst of the major attacks in the country.

The powerful blast in Kabul struck on a busy road during the morning rush-hour, shattering the windows of dozens of houses. The suicide bomber's body lay in the street near the   wreckage of his motorbike as police and intelligence officials cordoned off the area.

Karzai condemned the bombing, calling it "an act against humanity and against Islam."

NATO also condemned the attack, saying it had killed at least one intelligence service member and one civilian, and wounded at least 32 people.

The Taliban has proven resilient in the face of the US-led military coalition's nearly decade-long war. Although NATO poured more than 30,000 extra troops into the country last year to pressure the insurgents' traditional strongholds in the south, the Taliban have boosted their operations elsewhere, launching attacks across the north and east.

An extra contingent of 1,400 US Marines are to be deployed in the coming months in the southern province of Helmand, which along with neighboring Kandahar have seen some of the fiercest fighting.