The top U.S. military officer has arrived unannounced in Afghanistan to reassure a country rattled by a recent wave of high-profile attacks and assassinations.

Admiral Mike Mullen said Friday that U.S. officials are “not surprised at the spectacular attacks” and had predicted an increase in violence as the U.S. prepares to turn over control of security to the Afghan forces.

Mullen's trip comes as roadside bombs killed at least 19 civilians in southern Helmand province Friday.

Officials said at least 18 civilians – including children – died when their minivan hit an explosive device in Nahr-e-Saraj district. Another blast in Garmsir district killed one civilian and wounded four others when the farm tractor they were riding hit a mine.

No one claimed responsibility for the explosions, but officials say militants attacked security forces who responded to the minivan blast. It was not immediately clear if the responders suffered any casualties.

A recent United Nations report says violence against Afghan civilians has risen by 15 percent in the first half of this year, with more than 1,400 civilian deaths due mostly to insurgent attacks. The U.N. said roadside bombs were the single-largest killer of civilians in the first half of this year.

And while the Taliban typically does not claim attacks that cause significant civilian casualties, Friday's roadside bombings follow a coordinated Taliban assault on government buildings in central Afghanistan's Uruzgan province. That hours-long attack killed at least 19 people – including children and a journalist – and wounded 37 others.

The attack also follows a series of high-profile assassinations of key allies of President Hamid Karzai, as the Taliban attempts to disrupt the ongoing security transfer from the international coalition to Afghan forces in several parts of the country.

On Wednesday, the Taliban claimed responsibility for the suicide bombing that killed the mayor of the southern city of Kandahar. His death follows last week's targeted killing in Kabul of a senior advisor to President Karzai. And earlier this month in Kandahar, a trusted bodyguard shot and killed the Afghan leader's half-brother, Ahmad Wali Karzai.

Separately Friday, the international coalition said a roadside bomb killed two NATO soldiers in eastern Afghanistan.

Throughout the violence, U.S. and NATO forces have pressed ahead with transferring security control in the first seven areas of Afghanistan to Afghan forces.

About 33,000 American troops are set to leave Afghanistan by September of 2012, with all foreign combat troops scheduled to exit the war-torn country by the end of 2014.