Afghan security personnel and US soldiers inspect the scene of a suicide attack near Jalalabad airport on Friday.

AFP/Kabul

A Taliban suicide car bomb targeted a Nato convoy in eastern Afghanistan on Friday, killing at least three civilians as security forces brace for the start of the Taliban's traditional spring offensive.

Separately, 12 civilians onboard a minivan were killed on Friday when a roadside bomb struck their vehicle in the militant-infested southeastern province of Ghazni.

Taliban insurgents have stepped up suicide attacks on government and foreign targets since Washington announced a delay in troop withdrawals from Afghanistan last month.

The attack on the Nato convoy in Jalalabad city comes a day after 18 people were killed when Taliban insurgents mounted a six-hour gun and grenade siege on a courthouse in the usually tranquil northern city of Mazar-i-Sharif.

"In the morning today a foreign forces convoy came under a suicide attack near the airport in Jalalabad city," provincial police spokesman Hazrat Hussain Mashriqiwal told AFP.

He added that three civilians were killed and four others were wounded in the attack in Jalalabad, which is home to an important US military base.

Roadside bombs - like the one in Ghazni - have been a weapon of choice for the Taliban in their 13-year war against foreign and Afghan forces, though the militants seldom admit blame for attacks resulting in civilian casualties.

The Taliban, however, claimed responsibility for the Nato convoy in Jalalabad as Afghan forces brace for what is expected to be a bloody summer push by the insurgents.

The militants also said they were behind Thursday's terrifying assault in Mazar-i-Sharif, which underscored Afghanistan's precarious security situation as US-led foreign troops pull back from the frontlines after a 13-year war against the Taliban.

Growing attacks  

The uptick in attacks in recent days has taken a heavy toll on ordinary Afghans.

The number of civilians killed and wounded in Afghanistan jumped 22% in 2014, a recent UN report said, as Nato troops withdrew from combat.

The UN Assistance Mission in Afghanistan attributed the rise to an intensification in ground fighting, resulting in a total of 10,548 civilian casualties last year.

Nato's combat mission formally ended in December but a small follow-up foreign force has stayed on to train and support the local security forces.

President Barack Obama last month backpedalled on plans to shrink the US force in Afghanistan this year by nearly half, an overture to the country's new reform-minded leader, President Ashraf Ghani.

Hosting Ghani at the White House for their first presidential head-to-head, Obama agreed to keep the current level of 9,800 US troops until the end of 2015.

An American soldier was killed on Wednesday by an Afghan counterpart in eastern Afghanistan, the first insider attack since Obama's announcement.