KABUL, Afghanistan — The United Nations announced Saturday that May was the deadliest month for Afghan civilians since it began keeping count in 2007 — most likely a reflection of intense fighting, as militants seek to show they can stand up to the surge in American forces and try to undermine the government of President Hamid Karzai as it prepares to start taking over security.

A bomb killed two police officers and wounded nine others who were investigating an earlier explosion late Friday in the eastern province of Laghman, a spokesman for the provincial governor said.

Although the monthly record of 368 deaths is compared with 2007, it is effectively the highest number since the war began, because civilian casualties appear to have been far lower before then.

The majority of the casualties, 82 percent, were caused by Taliban and other militants, while 12 percent were caused by NATO troops and Afghan forces; in 6 percent of the cases, it was not clear who was responsible.

Even as the numbers were announced, a particularly deadly roadside bombing left 15 civilians dead in southern Afghanistan, among them 8 small children and 4 women.

Civilian casualties are a major concern of the United Nations, whose special representative to Afghanistan, Staffan de Mistura, has made a public effort for their reduction. The United Nations rarely announces monthly figures, but did so for May because the numbers were so high, officials said.

“The last time we saw figures like this was August 2010, but that was right before the parliamentary elections and typically we’ve seen increases in violence right before elections,” said Georgette Gagnon, the United Nations director for human rights in Afghanistan. “This time, there is no one event around which the violence is centered.”

The civilian deaths on Saturday occurred in Arghandab District of Kandahar Province when a large roadside bomb went off under a minibus carrying people from two related families. They were traveling to a famous shrine frequented by those seeking the healing of a child or family member.

The place where the attack occurred is an arid and mountainous area that lies near the border with neighboring Khakrez District, where the Taliban continue to be active, even though they were supposed to have been cleared out of Arghandab by NATO and Afghan troops last fall.

Security forces were also targets on Saturday, with a senior police officer and another officer killed by a suicide bomber in southeastern Afghanistan. The senior officer, Col. Mohammed Zahir, is the head of the quick-reaction force for Khost Province. The bombing was one in a string of attacks on senior police officers in the past several weeks.

Two police officers were also killed late Friday, by a roadside bomb in Mehtarlam, the capital of Laghman Province, which is scheduled to be among the first areas where Afghan security forces will begin to take full responsibility in a few weeks.

Two members of the NATO force were killed Saturday, one in an attack in southern Afghanistan and one in a helicopter crash in eastern Afghanistan, according to a NATO statement.

Taimoor Shah contributed reporting from Kandahar, Afghanistan, and Farouk Mangal from Khost, Afghanistan.