The international Red Cross warned on Wednesday of an increase in the number of Afghan armed groups operating in the country as bombings killed three children and a NATO soldier in separate attacks in southern Afghanistan.

NATO said the service member was killed in a blast early Wednesday in the south, while three boys - aged 10, 11 and 12 years - died in an explosion on the outskirts of Kandahar city, according to Khan Mohammad Mujhid, the police chief in Kandahar province.

Mr. Mujhid said a remote-controlled bomb, which was set up on a bicycle, exploded in a parking lot that serves as a gathering place for Afghans traveling to and from Saudi Arabia for the annual hajj pilgrimage.

“How could anyone attack this pilgrimage center?” asked Haji Alam, the father of two of the children who were killed. “I don’t know how a Muslim can do that?”

NATO said the attack also seriously wounded nine people, including two children and an Afghan policeman.

Tens of thousands of Afghan and NATO forces have been deployed in the southern provinces of Kandahar and Helmand and have been making progress in their battle against the Taliban. Militants have responded with bomb attacks aimed at threatening the population and preventing the government from gaining the support of the people.

Fighting also has continued in eastern Afghanistan, along the border with Pakistan, where the Haqqani network, a Pakistan-based Taliban faction closely tied to al-Qaeda, holds sway.

The Red Cross has long been able to operate in even the most dangerous areas of Afghanistan because of a commitment to treat all comers and because of its ability to negotiate access with the Taliban or other insurgent groups.

But a rise of small armed bands across the country has made such negotiations more difficult, said Reto Stocker, Afghanistan head for the International Committee of the Red Cross, or ICRC.

Often they don’t know who to talk with to ensure the safety of their workers and the shifting security situation makes it too dangerous to risk going into many areas, he said.

“In a growing number in areas of the country, we are entering a new, rather murky period. We see that the proliferation of armed groups threatens the ability of humanitarian organizations to access those in need,” Mr. Stocker told reporters in the capital, Kabul.

In the past, Red Cross workers were able to get out to areas where there had been clashes or violent incidents within days. Now, sometimes it takes weeks or months, he said.

“Access for the ICRC has, over the last 30 years never been as poor and difficult in Afghanistan,” Mr. Stocker said.

A number of humanitarian workers have been killed in attacks this year, including 10 members of a medical team that were ambushed in northern Afghanistan in August. The team included six Americans, two Afghans, one German and a Briton.

NATO did not disclose any details of the death of the service member. More than 670 U.S. and other international troops have died in Afghanistan so far this year.

Also on Wednesday, NATO said coalition and Afghan investigators have been sent to Helmand province to assess an incident in which coalition aircraft accidentally killed an Afghan civilian and wounded two children the day before. The aircraft was called in to support Afghan and coalition forces who came under fire Tuesday in Marjah district.

“We are here to protect the Afghan people and initial indications are that in this case we may have failed,” Brig. Gen. Tim Zadalis, the assessment team leader, said in a statement.

NATO expressed its condolences to the victims’ families.

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