Insurgents attacked American forces with rocket-propelled grenades and explosives on roads in southern Afghanistan (search) on Friday, injuring at least eight U.S. soldiers, two seriously, the U.S. military said.

The first attack occurred at 7 a.m. as a 10-vehicle convoy made its way along a road east of Daychopan, in southern Zabul province. About 10 suspected Taliban (search) insurgents fired RPGs at the convoy, prompting the American forces to fire back with small arms.

One of the RPGs struck a Humvee, injuring five troops, two of whom were being prepared for evacuation to a military hospital in Landstuhl, Germany (search).

About six hours later, rebels set off a roadside bomb near Zabul's provincial capital, Qalat, as a U.S. convoy passed. Three soldiers were injured, but all returned to duty.

It was not clear if there were any rebel casualties, said Maj. Rick Peat, a U.S. military spokesman.

In neighboring Uruzgan province, a convoy carrying election workers was also reportedly ambushed Thursday.

Four jeeps carrying staff from a U.N.-sponsored program to register voters for upcoming elections and their guards came under fire in remote Char Cheno district, provincial police chief Rozi Khan said.

He said the vehicles were destroyed and that two drivers and one election worker were missing, but had no further details.

Zabul and Uruzgan have been the scene of some of the worst fighting in recent months, and attacks have increased as the nation gears up for its first direct presidential election on Oct. 9.

The Taliban have vowed to disrupt the vote, and at least ten people helping prepare the election have been killed so far this year. Twenty-one American soldiers have also died in action this year, already the worst tally for the U.S. military since it entered Afghanistan in 2001.

The bloodshed has failed to dissuade Afghans from signing up to vote, with more than 90 percent of the estimated 9.8 million eligible already registered.

The presidential election, which interim President Hamid Karzai is expected to win, is to be followed by parliamentary elections in the spring. Both votes were delayed from June because of insecurity and logistical problems.