(Adds Swedish spokeswoman comments in paragraph 4)

By Abdul Saboor and Tahir Qadiry

KABUL, June 16 (Reuters) - Taliban suicide bombers launched a second day of attacks in Afghanistan on Saturday, killing at least four people in the capital and a northern city, raising tensions in areas that had been considered relatively secure.

A suicide car-bomber attacked a military-civilian convoy in Kabul on Saturday morning, killing at least three civilians and wounding five others, government and police officials said.

Hours later, two bombers riding a motorcycle attacked a military convoy in Mazar-i-Sharif, killing at least one Afghan civilian and wounding 15, a local security official said.

A Swedish military spokeswoman said the convoy was made up of mostly Swedish soldiers, as well as Finns and Poles. None of the 30 soldiers was wounded, she said.

The Afghan security official had earlier described the target as a Swiss convoy.

A spokesman for the Taliban, waging an insurgency against the Afghan government and its foreign allies, said the group was behind the attacks, which included two suicide blasts on Friday in south and central Afghanistan.

"Our many Taliban suiciders are present in the all cities in Afghanistan," Taliban spokesman Zabi-u-llah Mujahid told Reuters by satellite phone. "We will increase our suicide and guerrilla attacks...in coming days," he added.

The Taliban and their al Qaeda allies have adopted the tactics of Iraq's insurgency over the past two years, using suicide bombings, mostly aimed at foreign troops, to try to dispel the notion that foreign and Afghan forces are in control.

In Mazar-i-Sharif, seen as one of the most peaceful Afghan cities, the motorcycle bombers blew up as the convoy swept past on a stretch of road near a crowded vegetable market. It was the first suicide bombing in Mazar-i-Sharif in three years.

An eyewitness, Ahmad Jawad, said one of the bombers was wounded but appeared to have survived the blast.

"There were two people on a motorcycle and one of them was injured," Jawad said. "The cars passed and the suicide bombers exploded and the air filled with smoke."

Ahmad Zia, head of the provincial hospital in Mazar-i-Sharif, said some of the wounded were in critical condition and he expected the death toll to rise.

U.S. SOLDIER ACCIDENTALLY SHOOTS CIVILIANS

Amid the day's confusion and mayhem, a U.S. soldier shot and wounded two civilians near the scene of the Kabul blast, a U.S. military spokesman said. A senior police official said one of the civilians had died of his wounds in this incident.

"It appears ... that it was an accidental discharge ... and unfortunately it hit two Afghan civilians," the U.S. military spokesman said, adding the incident was under investigation.

Suicide bombings and civilian casualties from both sides in the Afghan conflict are raising security and political tensions and threatening to erode local support for foreign troops.

NATO-led and U.S.-coalition forces have more than 50,000 troops in Afghanistan and are under growing pressure to curb civilian casualties after a series of recent killings brought into question their tactics, such as aerial bombardment.

But NATO has blamed the Taliban, which ruled Afghanistan until U.S.-led forces invaded in 2001, for using civilians as human shields and sucking innocent people into the conflict.

A U.S.-coalition soldier was killed in central Uruzgan province on Saturday when a rocket-propelled grenade hit his vehicle, a U.S. military statement said.

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