Taliban suicide bombers have killed at least 16 people as they stepped up their fight against Afghan forces slowly taking the lead from US and international troops.—AP Photo

KABUL: Taliban suicide bombers have killed at least 16 people as they stepped up their fight against Afghan forces slowly taking the lead from US and international troops.

Attacks this week have included deadly bombings far from the main theatres in the south and east, underscoring that the Taliban and its allies retain the capability to strike over wide areas of the country. The violence comes at a pivotal time for the US-led coalition as it forges ahead with plans to hand over security responsibility to the Afghans.

The Taliban claimed responsibility for Tuesday's attacks—one in the western region of Herat and the other in the southern province of Helmand—in telephone messages to the media.

The Afghan army and police are now in charge of security for areas home to half the nation's population, with coalition forces in a support role. The coalition hopes to keep handing over control until Afghan forces are fully in charge by the end of 2013, with all combat troops scheduled to withdraw from the country by the end of 2014.

The US may retain a small number of forces past that date to help train and mentor the Afghan army and help with counter-terrorism efforts.

Afghan security forces now number about 330,000 and are to peak at 352,000 by the end of the year. They are expected to take over much of the fighting as the US draws down an additional 23,000 troops to 68,000 by the end of September. US troop levels reached a high of about 100,000 last year.

Military officials and Gen John Allen, the top US and coalition commander in Afghanistan, expect the Taliban to take advantage of the spring and summer fighting seasons to try to undermine the security transition.

Much of that fighting is expected to take place in the east, along Afghanistan's porous border with Pakistan. The Taliban retain safe havens in Pakistan's lawless tribal areas along the frontier, and insurgents regularly infiltrate into Afghanistan.

Coalition forces want to build on successes they had in the south last year. But they hope to consolidate those gains with fewer troops as they focus their attention on the east.

Attacks have been increasing since March 21, the first day of spring and the beginning of the Afghan calendar year.

According to an Associated Press count, 53 policemen and 13 Afghan army soldiers were killed in suicide attacks, gun battles and assassinations since that date. By comparison, 12 policemen and six soldiers were killed in the same period last year.

Some of the attacks have taken place in areas that are usually quiet, including the capital of northern Faryab province where a suicide bombing on April 4 killed at least 10 people, including three American soldiers.

The first and deadliest of Tuesday's attacks took place in Herat, a relatively peaceful province whose capital and many districts are already under Afghan security control.

A sport utility vehicle packed with explosives blew up outside the gate of the Guzara district police office as people were waiting to go inside to see government officials about various business matters.

The bombing came after police in the area received a tip that the black four-wheel-drive vehicle was loaded with explosives, said Raouf Ahmedi, a spokesman for the police commander of the western region of Afghanistan.

''They were chasing the car and tried to stop it,'' Ahmedi said. ''The vehicle then turned toward the district headquarters building and tried to pass the checkpoint, but police stopped them to be searched and asked where they were going.''

Moments later the vehicle exploded. The boom could be heard a few miles away. The remains of two men and a woman wearing a burqa were found inside the vehicle, officials said.

Three security officers and nine civilians were killed, and more than 50 people, including children, were wounded, said Nasar Ahmad Popul, the Guzara district chief who was inside the headquarters at the time of the blast.

In southwest Helmand, three suicide attackers wearing vests laden with explosives parked their car outside the police office and walked toward the entrance, said Daud Ahmadi, a provincial spokesman.

Police fired at the attackers, killing one. Two others blew themselves up inside the compound, Ahmadi said. He initially reported that eight policemen died but later revised the toll, saying that four policemen were killed and five, including the district commander, were wounded.