ABUL, Afghanistan, Aug. 13 — In the most violent 24-hour period in Afghanistan in nearly a year, 15 people, including 6 children, were killed when a bomb exploded on their bus in southern Afghanistan. More than 40 others were killed today in fighting in the country's east and south.

The bomb exploded in Helmand Province aboard a bus headed for the provincial capital, Lashkar Gah, according to wire reports. It was the deadliest such attack since a bomb exploded in Kabul last September, killing 35 people.

The bombing in Helmand and a series of other attacks today, on top of other recent setbacks, all gave notice to the American-backed government of the growing threat to the nation's stability.

In the east, suspected Taliban guerrillas attacked government soldiers in the province of Khost, about four miles from the border with Pakistan, late Tuesday night. Fifteen attackers were killed, as were five government soldiers, according to a spokesman for the provincial governor quoted by The Associated Press.

A local commander said that government troops had captured one Pakistani guerrilla and one Arab whose nationality was unknown.

Afghan officials have accused Pakistan of allowing Taliban insurgents to operate unimpeded and to make forays into Afghanistan.

In Oruzgan Province, a clash between the forces of rival warlords loyal to the government of Hamid Karzai left more than 20 fighters dead, one of the commanders, Haji Abdul Rahman, told The Associated Press.

In Kabul today, two university students were killed and one was seriously wounded when a bomb they were making went off by accident, the police said. In June, four German peacekeepers were killed by a suicide bomber in the capital.

The violence cast into gruesome relief the growing threats to the country's stability from what are believed to be remnants of the Taliban or Al Qaeda, as well as Afghans opposed to the American-backed administration led by Mr. Karzai.

The attacks came two days after the North Atlantic Treaty Organization, in a historic departure from its traditional European theater of operations, assumed control of the International Security and Assistance Force, the multinational peacekeeping force that patrols Kabul and the areas surrounding it.

And they came as the United States was preparing to invest another $1 billion in Afghanistan, possibly supplemented by another $600 million from other countries, in an attempt to accelerate the pace of reconstruction. A significant amount of the aid, according to Afghan officials, will be devoted to strengthening national institutions — particularly the national army and police — that could help provide security outside Kabul.

Warlords remain entrenched around the country, and Afghanistan is once again the world's largest opium producer. Afghan officials say they fear the opium trade, which both profits from and feeds the insecurity around the country, could pull the country under.

Stepped-up attacks in the southeast, meanwhile, including some on aid workers, have prompted aid groups to restrict their movements and work in a region that is already very poor.

Last weekend, the United Nations suspended road travel for its workers in southern Afghanistan after several Afghan aid workers were tied up and beaten. Last week six Afghan soldiers and a driver for the aid group Mercy Corps were killed in an attack on an office in Helmand.

In the neighboring province of Kandahar, which was the wellspring for the Taliban movement, two pro-government clerics were killed and a third wounded after a clerics' council they served on urged Afghans to support Mr. Karzai's government.

But while the attacks on aid workers, clerics and soldiers seemed intended to deter those supporting the government or rebuilding or securing the country, the attack on ordinary Afghans represented by today's bus bombing may mark a new stage in the violence in the south.

"They are killing innocent people," the deputy governor of Helmand, Haji Pir Muhammad, said, attributing the attack to Al Qaeda and the Taliban, according to The Associated Press. Local officials said the bus, which was carrying 20 people, was totally destroyed. The surviving passengers and the driver were wounded.

The aim could be to make Afghans feel betrayed by a government unable to protect them, or to enhance the appeal of an alternative that promises stability and order — exactly what the Taliban represented in a country torn by civil war when they took power in the mid-1990's.

The attack in the east came a day after the third meeting of a commission composed of representatives from Afghanistan, Pakistan and the United States intended to reduce tension and resolve border disputes between the two neighbors and coordinate the campaign against terrorism.

The first item discussed by the commission, according to Omar Samad, a Foreign Ministry spokesman who was present, was the killing of two Pakistani border guards by American forces pursuing attackers on Monday. The incident is under investigation, Mr. Samad said, but in the meantime the three countries decided to set up a three-way hot line linking senior representatives.