Afghan forces tightened security in Kabul on Tuesday, a day after a brazen Taliban assault on the city center left 12 people dead and raised concerns about the government's ability to protect the people.

Troops searched vehicles entering the capital and increased the number of checkpoints in the city, along with foot patrols and vehicle patrols, said deputy police chief Mohammad Khalil Dastyar.

The streets otherwise were calm and traffic was back to normal levels a day after the attack, which sent terrified Afghans racing for cover as explosions and machine-gun fire echoed across the city. Afghan forces along with NATO advisers managed to restore order after nearly five hours of fighting.

The assault by a handful of determined militants dramatized the vulnerability of the Afghan capital, undermining public confidence in the ability of the government and its U.S.-led allies to provide security. A handful of gunmen -- perhaps fewer than a dozen -- paralyzed a city of 4 million for hours, forcing government ministries to stop work as police used ministry buildings as firing positions.

Shopkeepers shuttered their stores as rumors of suicide bombers roaming for targets swept the city.

Mohammad Nasir, a taxi driver, said the government needs to do more to stop foreign attackers from crossing the border.

"They always say that these attackers are coming from outside, but they don't have wings to fly from the sky and come here, so they come from the ground," he said. "If we had professional Afghan forces, they could stop them ... but we see we don't have professional forces to keep them from coming."

NATO insisted the operation to secure the capital on Monday was Afghan-led, though it had explosives experts and other troops in supporting roles.

New Zealand's Prime Minister John Key said Tuesday that some of his country's troops were caught up in the fight.

"A small element of the SAS (special air service commandos) were among those who took up positions close to the incident," New Zealand Prime Minister John Key told reporters, adding their involvement was very limited. "They were quite a long way back from the building (where insurgents holed up) as I understand it."

New Zealand has five soldiers attached to the headquarters of NATO's International Security Assistance Force in Kabul and another 140 troops involved in provincial reconstruction and security in Bamiyan province northeast of Kabul.

Other NATO troops were on the ground Monday helping the Afghan forces with bomb disposal expertise, Lt. Nico Melendez, a NATO spokesman, said in an e-mail. He said Turkish troops were assisting in the city Tuesday as well but their presence was normal.

NATO military chief Adm. James Stavridis, who arrived in Kabul for a visit Monday as the attack was under way, lauded the Afghans for successfully containing the violence.

"Afghan national security forces effectively countered an insurgent attack in downtown Kabul," he said in a statement, adding the coordinated effort "highlights their improved effectiveness in protecting Afghan citizens."

The attacks also sent a message that the mostly rural Taliban are prepared to strike at the heart of the Afghan state, bringing the city to a standstill, even as the United States and its international partners are rushing 37,000 reinforcements to join the eight-year war.

It was the biggest assault on the capital since Oct. 28, when three gunmen with automatic weapons and suicide vests stormed a guest house used by U.N. staff, killing at least 11 people including five U.N. workers. Taliban suicide bombers and gunmen also struck government buildings in the capital last February, killing more than 20 people.

But Monday's fighting persisted longer than any initiated by the Taliban in the capital since they were driven from power by the U.S. and its allies in 2001.

"The government should maintain good security at the four main entrances of Kabul city and should not let the suicide attackers and bad people enter our land and kill innocent people," said Mohammad Amin as he stood in front of a charred shopping center that was destroyed Monday after two suicide bombers detonated their vests inside during a gunbattle with Afghan troops.

Elsewhere, four Afghans who had been kidnapped along with two Chinese engineers were released Tuesday, three days after they were seized by Taliban militants on their way home from working on a road construction project in northwestern Afghanistan, an official said.

Afghan and NATO forces in the Ghormach area of Faryab province also killed 10 Taliban militants and detained three others late Monday in an operation linked to the kidnapping, the deputy provincial governor, Abdul Sattar Barez, said without elaborating.

Barez said the four Afghans were released with the help of village mediation and authorities were optimistic the Chinese men would be freed soon.

Four militants also were killed Monday afternoon in clashes with police in Farah province, according to Raouf Ahmadi, a police spokesman for western Afghanistan.