KABUL: Five American troops were killed in attacks in southern Afghanistan, where the U.S. and NATO have ramped up operations against the Taliban and seen casualties rise quickly in what has been the deadliest year of the war for international forces.

Four soldiers died Thursday in the same small district of Zabul province, including three killed when their Stryker vehicle struck a bomb, said U.S. military spokesman Lt. Robert Carr. The fourth was shot to death in an insurgent attack, Carr said. The Stryker brigade in Zabul is part of the influx of U.S. troops sent by President Barack Obama over the summer to try to reverse Taliban gains.

A U.S. Marine was fatally shot while on foot patrol in Nimroz province and died Thursday, said Capt. Elizabeth Mathias, a military spokeswoman.

The deaths come as the Obama administration debates whether to send still more forces to Afghanistan.

The Pentagon said the U.S. and NATO commander in Afghanistan, Gen. Stanley McChrystal, would ask this week for additional American forces — a number that officials said could reach as high as 40,000 troops. But some question the wisdom of sending more troops to support a government facing allegations of widespread fraud from the disputed Aug. 20 presidential election.

Bombs planted in roads, fields and near bases now account for the majority of U.S. and NATO casualties and have proven especially dangerous in the south, which is largely controlled by the Taliban.

With the five deaths, a total of 34 U.S. forces have died in Afghanistan in September. August, which was the deadliest month of the war for American troops, saw 51 deaths.

In his report to the White House, McChrystal argued that military commanders should worry less about protecting their own forces and get out into Afghan communities. Although he acknowledged this "could expose military personnel and civilians to greater risk in the near term."

"Accepting some risk in the short term will ultimately save lives in the long run," he wrote.