Taliban Plans to Increase Attacks .

KABUL—The Taliban Saturday promised to step up insider attacks by Afghan infiltrators on U.S. and coalition troops as the insurgent movement announced the beginning of its annual spring offensive.

In a statement, the insurgent group's leadership said Taliban spring operations would begin Sunday, with a focus on attacks against foreign troops and diplomatic installations.

"Successful insider attacks, to eliminate foreign invaders, will be carried out by infiltrating Mujahideen inside enemy bases in a systematic and coordinated manner," the statement said.

Insider attacks have proven a formidable tactic by insurgents to drive a wedge between coalition troops and their Afghan allies. Last year, so-called "green-on-blue" attacks in which Afghans in military or police uniform turned their weapons on coalition troops surged. At least 61 coalition troops were killed in such attacks in 2012.

The Taliban timed the start of their offensive to coincide with the national holiday in Afghanistan that marks the victory of Afghan Mujahideen over the Soviet-backed regime.

In the statement, the Taliban also called on employees of the government of Afghan President Hamid Karzai to quit their posts.

"We once again call on all the officials and workers of the stooge Karzai regime to break away from this decaying administration in order to conform to Islamic commands, national interests and protection of yourselves and to choose a life of prosperity living alongside your own people in an atmosphere of peace and security," the statement read.

This year's fighting is shaping up as a crucial test for Afghan security forces, which are due this spring to take lead responsibility for security around the country. U.S. and coalition combat troops are scheduled to complete their withdrawal from Afghanistan by the end of 2014, although discussions are under way between Kabul and Washington about a small coalition military force that will conduct training and limited counterterrorism operations after 2014.

In recent years, insurgents have staged headline-grabbing attacks to coincide with the beginning of Afghanistan's fighting season, which begins with the spring thaw and the return of Taliban fighters from winter havens across the border in Pakistan.

Last April, insurgents staged coordinated attacks in Kabul and across Afghanistan, striking foreign embassies and military bases. And in May 2011, the Taliban launched a major assault on Kandahar, southern Afghanistan's largest city, attacking government buildings and security installations.

While levels of violence typically drop in the winter, insurgents have managed to strike inside the capital in recent months. In January, suicide attackers launched a failed bid to breach a fortified compound of the National Directorate of Security, the country's spy agency. And in early March, a bomber struck outside the Afghan Ministry of Defense during a visit to Kabul by U.S. Defense Secretary Chuck Hagel.