KABUL, Afghanistan — Two explosions in eastern Afghanistan, one targeting an American convoy, killed three U.S. troops, an Afghan interpreter and at least 24 other Afghans on Wednesday, defying what the military had described as a trend of diminishing violence this year.

Western officials had been citing decreased civilian casualties in the first four months of the year as a sign that the insurgency is waning and Afghan forces are increasingly showing the ability to safeguard the country. But deaths have been spiking in recent weeks.

Both blasts — one in the city of Khowst, the other in a rural district of Lowgar province — occurred at the lunch hour, a time when many people are on the streets.

The Khowst explosion, aimed at a convoy carrying coalition troops, was triggered by a suicide bomber on a motorbike, the Interior Ministry said. The ministry put the Afghan death toll at 16, including two police officers, and the injury count at 37, including two women.

Western military officials said three Americans and a translator died in the explosion, and the U.S. Embassy condemned what it called a “murderous campaign against all” by the Taliban and other insurgents.

Khowst, the capital of the province of the same name, lies just across the border from Pakistan’s tribal areas, where the Haqqani network, a virulent Taliban offshoot, is based. The Haqqanis are active in Khowst and neighboring provinces, but other insurgent groups operate there as well. There was no immediate claim of responsibility.

Hospitals and clinics in Khowst were overwhelmed by the number of casualties, and besieged by panicked relatives of those who were injured and killed.

Khowst city remains volatile even though there is a major American-run base on its outskirts. That installation, known as Camp Salerno, came under a fierce and concerted insurgent attack on June 1 that left dozens of troops seriously injured.

At the time, the NATO force disclosed little about the incident, including the fact that insurgents had set off a huge truck bomb at the gates of the installation, causing about 100 injuries, some three dozen of them serious. The Washington Post first reported the actual severity of the attack, an account that was subsequently confirmed by Western military officials.

About the same time as Wednesday’s Khowst attack, a civilian vehicle in the Baraki Barak district of Lowgar province hit a roadside bomb, killing eight civilians, half of them children, the Interior Ministry said. It blamed “terrorist Taliban” for planting the device.

Two other civilians were hurt in that blast, the ministry said.

Baraki Barak was the scene of a recent civilian-casualty episode that inflamed tensions between the government of Afghan President Hamid Karzai and the NATO force. An American airstrike on a civilian compound where a wedding had taken place killed at least 17 civilians early this month, together with some insurgents.

Western military officials initially denied any civilians had died, but days later acknowledged the deaths, and Gen. John Allen, the commander of Western forces in Afghanistan, traveled to Lowgar to extend condolences and apologies.

Los Angeles Times special correspondent Hashmat Baktash contributed to this article.