A bomb at an Afghan hospital on Saturday killed at least 20 people including women and children, days after US President Barack Obama said 10,000 US forces would leave the country this year.

The brazen suicide car bombing in Logar province, about 75 kilometres (45 miles) south of the capital Kabul, also wounded over 20 people and officials warned the death toll could still rise.

It was condemned ‘in the strongest terms’ by the United Nations, which said the hospital's maternity ward was badly damaged in the attack and many of the victims were women and children.

Underlining the confusion and chaos at the site, the ministry of public health initially put the death toll at 60 but later corrected its own figures.

An eyewitness described horrific scenes of victims on fire and body parts scattered in all directions following the blast in the remote district of Azra, close to the border with Pakistan.

"At least 20 of our countrymen have been martyred and around 25 are wounded," the ministry said, strongly condemning the attack.

The Afghan interior minister also said that 20 people were killed including hospital staff and patients, while 23 civilians were wounded.

But provincial health director Mohammad Zaref Nayebkhail said the death toll could still be significantly higher as many people came to the scene quickly after the blast and removed the bodies of their relatives.

Din Mohammad Darwaish, the Logar provincial spokesman, said the attack was a suicide car bombing.

One man who lives near the devastated hospital, Abdul Rahman, said that he lost seven relatives in the explosion.

"Seven members of my family including three women and two children went to that hospital this morning," he said, through tears.

"I was at home, then I heard a big explosion. When I rushed to the site, I saw many dead and injured people.

"Many of them were burning, on fire. There were body parts everywhere. My family is dead, I can't find them, they are under the rubble."

The Taliban denied it was behind the attack, with spokesman Zabihullah Mujahid saying: "We condemn this attack on a hospital... whoever has done this wants to defame the Taliban."

Staffan de Mistura, the special representative of the United Nations secretary-general for Afghanistan, called the attack "despicable."

"Much of the damage was in the maternity ward of the hospital, and many of those killed and injured were women and children," he said in a statement.

The United Nations Assistance Mission in Afghanistan (UNAMA) also stressed that attacks on hospitals were strictly prohibited under international humanitarian law.

The blast came at the end of a week when Obama announced that 33,000 US forces would leave Afghanistan by the end of next summer.

All foreign combat forces are due to pull out of the country by the end of 2014. There are currently up to 150,000 foreign forces in Afghanistan, including about 99,000 from the US.

Some analysts fear that Afghan security forces may struggle to contain the insurgency, which has hit record levels of violence, as withdrawals begin.

Afghan President Hamid Karzai condemned the attack as "savage and ignorant" in a statement released by his office.

It came as Karzai told a counterterrorism summit in Tehran that militancy was on the rise in both his country and the region.

"Not only has Afghanistan not yet achieved peace and security but terrorism is expanding and threatening more than ever Afghanistan and the region," he told the opening session.

The two-day summit is being attended by the heads of state of six regional countries, including Afghan neighbours Iran and Pakistan.

The blast in Logar is the second major attack in Afghanistan in two days.

On Friday, 10 people were killed by a bicycle bomb which went off in a busy bazaar in Khad Abad district of the northern province of Kunduz.

Militants in Afghanistan frequently target the Afghan police and other government employees as well as foreign forces in their near decade-long insurgency.

But civilians are the biggest casualties in the war, with 2,777 killed last year, according to the United Nations.