Heavily armed government troops thronged the streets of Afghanistan's capital Thursday, stepping up security before the arrival of the new U.S. envoy a day after Taliban attacks showed how easily the city's defenses can be breached.

Richard Holbrooke, President Barack Obama's recently appointed envoy to Afghanistan and Pakistan, was expected later in the day for his first trip to the country. Security would have already been high for such a visit, but Holbrooke arrives following one of the Taliban's most audacious attacks on the capital.

In the east, meanwhile, a suicide bomber Thursday blew himself up outside a police station in Sharan, the capital of Paktika province, killing an officer and wounding 10 others, the Interior Ministry said in a statement.

The Taliban regularly target Afghan and foreign troops with suicide and roadside attacks, and other violent incidents have already spiked this year.

On Wednesday, Taliban militants killed 20 people in a coordinated assault on three government buildings. Armed with guns, grenades and suicide vests, they stormed through barricades at the Justice Ministry in the heart of Kabul and a corrections department building to the north.

One attacker was killed before he could force his way into a third building, the Education Ministry.

The Taliban claimed responsibility soon after the assault began.

The attack served as a reminder of the challenges facing Obama as he increases America's focus — and troop levels — in Afghanistan. The new administration has promised up to 30,000 new troops. Holbrooke is helping the Obama administration chart a new strategy to beat Taliban insurgencies in both Afghanistan and Pakistan.

Under rain and snow, troops armed with heavy machine guns swarmed street intersections in Kabul on Thursday, checking drivers' papers and searching cars.

"Security measures have been increased 100 percent, particularly at the gates of Kabul," said Abdul Gafar Pacha, the head of the police criminal investigation unit.

All eight attackers died in Wednesday's assaults, bringing the death toll to 28. Another 57 people were wounded, according to the Interior Ministry.

Amrullah Saleh, the head of Afghanistan's intelligence service, said the attackers sent text messages to a militant leader in Pakistan before the attack.

Afghanistan has accused militants based in Pakistan's lawless tribal areas or Pakistan's spy service of being behind several major attacks in Kabul, including the bombing of the Indian Embassy last July, an assassination attempt against President Hamid Karzai in April and an assault on the luxury Serena Hotel in January 2008.