KABUL, Afghanistan - The U.N. on Monday accused militants and criminals of killing 34 aid workers in Afghanistan this year and urged armed groups to stop attacking humanitarian convoys so food can reach millions of poor Afghans.

Underscoring the country's increasing violence, a six-hour battle in the south left more than 50 militants dead or wounded, while a roadside bomb killed a U.S.-led coalition soldier in the same region.

The U.N. plea comes as Afghanistan is in one of the most violent periods since the U.S.-led invasion in 2001. More than 5,300 people have died this year in insurgency-related violence, according to an Associated Press count based on figures from Afghan and Western officials.

Insurgents and criminal gangs have killed 34 aid workers this year, abducted 76 others and attacked or looted 55 aid convoys, said Tom Koenigs, head of the U.N. Assistance Mission to Afghanistan.

"The attacks on humanitarian aid must stop," he said. "Those responsible for these attacks and for the insecurity are pushing the most vulnerable people outside of our reach. Those responsible for these attacks need to know that they are attacking the welfare of Afghanistan's most vulnerable communities."

The majority of the aid workers attacked in 2007 were Afghan nationals, including doctors, engineers and land mine removers, the U.N. said.

Rick Corsino, country director for the U.N.'s World Food Program, said 30 WFP food convoys have been attacked this year, mainly in the south, compared with five attacks in 2006.

"In a majority of these incidents, food was looted ... and so far we have lost something like 1,000 tons of food," Corsino said.

Violence in southern Afghanistan has prevented the WFP from moving any aid convoys the last six weeks on the highway between the biggest southern city, Kandahar, and the major western city, Herat, he said.

Authorities have six weeks to get food to about 400,000 Afghan living at high elevations before winter sets in, Corsino said. Nearly 5 million Afghans rely on food aid, and the WFP has already distributed 220,000 tons of food worth $150 million this year, he said.

The fighting in the south saw NATO-led forces and Afghan troops launch an attack on Baluch village in southern Uruzgan province during a gathering of local Taliban on Sunday, Afghan authorities said.

"More than 50 enemies were killed or wounded" and 13 others detained during the joint operation, the Defense Ministry said.

Maj. Charles Anthony, a spokesman for NATO's International Security Assistance Force, said that "several dozen militants were killed."

Separately, a roadside bomb in the Sangin district of another southern province, Helmand, killed a soldier in the U.S.-led coalition and wounded another as they helped deliver supplies for the Afghan army Monday, a coalition statement said. Their nationalities were not released.

Elsewhere in Helmand, a suicide bomber blew himself up next to a taxi stand in the provincial capital, Lashkar Gah, killing three civilians and a policeman, the Interior Ministry said. Six people were wounded.

Continuing an Afghan government campaign against unregistered private security firms, Afghan police raided and shut down a British-based security company in Kabul on Monday.

Police arrested three Afghan guards and the Afghan director of Olympus Security Group for operating without a license, the eighth such firm to be closed this month.

"Today was No. 8. Tomorrow will be No. 9," said Ali Shah Paktiawal, director of criminal investigations for the Kabul police. Paktiawal said previously that 12 or 13 security companies would be targeted.

Olympus, which has only a small presence in Kabul, was the first foreign company to be shut down following closures of seven Afghan firms. A woman who answered the phone at the company's headquarters in England said no one was available to discuss the Afghan operation.

Officials say some of Kabul's security firms are suspected of involvement in criminal activity such as killings and robbery. About 60 security companies are registered with the government, but two dozen more are thought to be operating.

In Tokyo, Japanese officials said its supply support for coalition naval forces involved in the Afghanistan war effectively ended Monday, at least for now.

A Japanese tanker refueled a coalition warship in the Indian Ocean and there were no further plans to refuel ships before the law authorizing the mission expires Thursday, said a Defense Ministry official who spoke on condition of anonymity, citing protocol.

Japan, America's top ally in Asia, has refueled coalition warships since 2001, and U.S. officials are urging the mission be extended. But the attempt by Japan's pro-U.S. ruling coalition to renew the deployment has been stalled in parliament by a resurgent opposition.

Associated Press writers Jason Straziuso and Rahim Faiez in Kabul and Noor Khan in Kandahar contributed to this report.