JALALABAD, Afghanistan -- The Taliban unleashed a wave of attacks that killed 21 people across Afghanistan in a show of strength to start the spring and a new test for Afghan security forces.

Fighting in Afghanistan escalates in the spring each year with the end of snowy winter weather, which hampers movement. The snow's melting opens up mountain passes that allow militant forces to move in from refuges in neighboring Pakistan.

This year's offensive by the Taliban will be an important gauge of how well Afghan government forces face insurgent attacks once foreign combat forces leave at the end of the year. Since last spring, Afghan troops and police have taken up full security duties in the country, with U.S. and NATO troops training and mentoring in the background, rarely intervening directly with air support.

About 30,000 U.S. troops remain on the ground in Afghanistan, the lowest number since the 2001 invasion.

The Taliban warned last week that they would launch their annual spring offensive on Monday, and it was no bluff -- with a flurry of rocket blasts and attacks on police around the country and the storming of a government building in an eastern city.

The violence began with a predawn rocket attack on the main NATO base at Bagram, just north of the Afghan capital, and another on Kabul's international airport soon after. Neither caused much damage.

But about 9 a.m., three Taliban militants attacked a provincial Justice Ministry building in Jalalabad in eastern Afghanistan. They shot and killed two police guards and rushed into the building just as employees were arriving for work, said provincial government spokesman Ahmad Zia Abdulzai.

Security forces rushed to the scene, including three armored vehicles that took up positions around the two-story concrete building. Gunners on the vehicles blasted the building with heavy machine guns from several directions during a 4½-hour gunbattle with the militants inside.

By the end, the building was devastated, with burned-out offices and smashed furniture, an Associated Press photographer who entered the building said.

Five civilians were killed, including three elderly employees in the office, and seven others were wounded in the battle. Once they retook the building, police found two dead attackers along with the remains of a third who police said died when he detonated an explosive vest.

Taliban spokesman Zabihullah Mujahid claimed responsibility for the Jalalabad attack. He said the assault was retaliation for what he called harsh rulings by the Justice Ministry against the Taliban.

With foreign forces in the background, Afghan troops have faced the brunt of militant attacks in the past year. In 2013, insurgents killed almost as many members of the Afghan security forces as they lost themselves, according to a report released Monday by the International Crisis Group. It cited an estimate by a "western security analyst" of 9,500 insurgents killed or wounded last year, compared with 8,200 members of the Afghan security forces.

"With less risk of attack from international forces, they are massing bigger groups of fighters and getting into an increasing number of face-to-face ground engagements" with Afghan forces, the group said in its report.

This year's Taliban offensive comes at a sensitive time, against the backdrop of a key presidential election. Militants have already stepped up attacks to sow insecurity during the voting. Final results of the first round of the election, held April 5, are to be announced Wednesday. The two top vote-getters are expected to face a runoff later this month.

In the other attacks Monday, insurgents killed nine policemen in an attack on a checkpoint in southern Helmand province, Gov. Sulaiman Shah Sarwani said. North of Kabul, a rocket hit a market in Parwan province, killing two civilians and wounding four. The Taliban also claimed responsibility for that attack.

Elsewhere in the east, groups of militants, including some on motorbikes, attacked police checkpoints on the outskirts of the city of Ghazni, killing two women and a policeman and wounding eight people, said the provincial governor, Mohammad Ali Ahmadi.

No one immediately claimed responsibility for the Ghazni attack.

Elsewhere in Jalalabad, attackers targeted a police vehicle and detonated a roadside bomb, wounding six people, including two policemen.

Information for this article was contributed by Amir Shah and Greg Keller of The Associated Press.

A Section on 05/13/2014