Three of the six Red Cross aid workers abducted by gunmen in northwest Syria on Sunday have been released, along with a volunteer from the Syrian Arab Red Crescent, the International Committee of the Red Cross (ICRC) said today.

However, there was no word Monday on the fate of the other three whose kidnapping highlights the risk to  continuing humanitarian work in a country fragmented by war.

Robert Mardini, head of ICRC operations for the Near and Middle East, said in a tweet the four were "safe and sound" 
after their abduction on Sunday.

ICRC spokesman Ewan Watson said they had been released in the Idlib region, a near-lawless area in northeast Syria where hundreds of militia operate, but did not elaborate on the circumstances.

The ICRC was awaiting information on the remaining three, he said. The six Red Cross workers and local Red Crescent volunteer were abducted by unidentified gunmen as they were returning to Damascus after a four-day mission to deliver medical supplies.

"Of course this type of incident is terrible because it is disruptive and puts in jeopardy our operations in Syria," Mardini told Reuters in Geneva hours before the partial release.

The ICRC remain committed to its relief operations in Syria where it is delivering food, water and medical supplies to displaced civilians and trying to evacuate the wounded, he said.

Two and a half years into the civil war that grew out of a crackdown on anti-government protests, pro- and anti government militia have fragmented the country into small fiefdoms.

Kidnappings of civilians, aid workers and journalists have spiked this year as groups linked to al-Qaeda and opportunist criminals have exploited the vacuum of power.

Some aid agencies have had to adapt their work, others are scaling back.

"The security situation has got much worse in recent months, especially in August, given the rise of the influence of 
extremist groups directly linked to al-Qaeda," said Jitka Skovránková, who works for the Czech People in Need, one of the few aid groups working in Aleppo city, in north Syria.

She said fighting between al-Qaeda's linked groups and other rebels as well as Kurdish groups along the Turkish border had made her organisation change the way aid enters Syria.

"There are now no open official border crossings for humanitarian cargo in the areas of Idlib and Aleppo governorates, where we work," she said, adding that People in Need works with the Turkish border police and the Turkish Red  Crescent to deliver aid through semi-official crossings, often dirt roads on other parts of the frontier to get aid in.

Foreigners stay away

The growing risk of kidnappings means that their foreign staff have reduced their presence in Syria, she says. 

"Previously, our foreign staff spent 80 per cent of their time in Syria. Now they haven't been inside for four weeks.

People in Need continues to provide aid to Syrians, relying on their local staff to deliver it, but Skovránková says the rapidly changing situation is making travel preparations and aid delivery harder. "You have to be aware. It's not slowing the operation but it's more difficult to get the operation done."

Other aid groups have also adjusted operations to the changing security situation.

Simon Ingram, a UNICEF spokesman, said that the UN Children's Fund is ""trying to operate in a more localized manner than we might have done in the past."

Aid workers say the Damascus government has also placed barriers on working in Syria such as rejecting visas or preventing convoys from entering certain areas.

Syrians complain that aid is not getting through to many areas of the country, especially to residents living in rebel-held territory who say the government is restricting access.

Opposition activists say that rebel-held towns around the capital have been besieged by the army and say food and medicine are in very short supply.

Syrian authorities evacuated 5,000 women and children from the town of Mouadamiya over the weekend, state media said. The ICRC says around 10,000 civilians are still in the area, which opposition activists say has been besieged by the government for months.

World powers fail to halt violence

The ICRC's U.K. spokesman, Sean Maguire, says the group has "good contacts within the layers of government authorities and the armed opposition."

But, he said, "the armed opposition are very fractured and we have to deal with groups of different shades". As territory changes hands between different armed groups, it is hard to identify who is in control of certain areas.

Divided world powers have failed to halt the violence.

U.S. Secretary of State John Kerry called on Monday for a peace conference on Syria "very soon" but said peace would not be possible without a transition government to replace Syrian President Bashar al-Assad. 

While Russia backed a June 2012 agreement calling for the creation of a transitional government, it says Assad's exit from power must not be a precondition for talks on peace or a political solution.

More than 100,000 people have been killed and the United Nations says that half of Syria's 23 million people will need humanitarian aid by the end of 2013. More than two million have already fled to neighbouring countries and millions more have been forced to move, some multiple times, inside Syria.