KABUL — Five American troops were killed within 24 hours in southern Afghanistan, where Taliban militants have conducted an unrelenting campaign of bombings and attacks against U.S. and NATO forces.

This has been the deadliest year of the war for international forces, and the Obama administration is debating whether to add still more troops to the 21,000-strong influx that began pouring into the country over the summer.

The five deaths announced today occurred in three separate attacks the day before in the south, where U.S. and NATO commanders have ramped up their operations to try to reverse Taliban gains.

The commander of U.S. and NATO forces, Gen. Stanley McChrystal, told "60 Minutes" the strength of the militant group took him by surprise when he arrived this summer.

"I think that in some areas that the breadth of the violence, the geographic spread of violence, is a little more than I would have gathered," he said in the interview, to be broadcast on Sunday.

Four soldiers died in the same small district of southeastern Zabul province, three of them killed when their Stryker vehicle hit a bomb, and the fourth shot to death in an insurgent attack, said U.S. military spokesman Lt. Robert Carr. The Stryker brigade arrived in Zabul as part of the summertime surge to try to secure the region ahead of Afghanistan's Aug. 20 presidential election.

Meanwhile, a U.S. Marine was fatally shot while on foot patrol in southwestern Nimroz province, said Capt. Elizabeth Mathias, a military spokeswoman.

The U.S. is on track to have 68,000 troops in Afghanistan by the end of the year, but the Pentagon said McChrystal would ask this week for as many as 40,000 new forces. Some question the wisdom of sending more troops to support a government facing allegations of widespread fraud in last month's disputed vote.

In a report to the White House, McChrystal argued military commanders need to be less preoccupied with protecting their troops and send them out into Afghan communities more. He acknowledged this "could expose military personnel and civilians to greater risk in the near term," but said the payoff in terms of forging ties with the Afghan people would be worth it.