Two roadside bombs that struck back-to-back only miles apart hit two U.S. military vehicles Monday, killing four American troops, U.S. and Afghan officials said.

U.S. military officials have predicted a 50 percent spike in IED attacks in Afghanistan this year as militants step up attacks and thousands of more troops pour into the country.

The deaths bring to 64 the number of U.S. forces killed in Afghanistan this year, according to an Associated Press count based on military figures. That far surpasses the 36 troops killed through the first five months of 2008, the deadliest year for American forces since the 2001 U.S.-led invasion to oust the Taliban.

Col. Greg Julian, the top U.S. spokesman in Afghanistan, said the two IED strikes occurred "not too far away from each other." He declined to give further details, including where the strikes occurred, because family members hadn't yet been notified. He confirmed the deaths of four Americans.

"We certainly do expect an increase and there has been an increase in insurgents' use of IEDs, which is their primary means of conducting attacks because they can't launch an effective direct attack," Julian said. "And we anticipate a continued increase in the use of this tactic."

Though the U.S. military declined to say where the strikes occurred, an Afghan official said the two attacks happened in Wardak, one province west of Kabul that saw an influx of 10th Mountain Division troops earlier this year.

Mohammad Hanif Hanifi, the top government official in Nirkh district, said a U.S. vehicle hit an IED around 6 p.m. in a village that troops routinely patrol through around 6 p.m. He said a second vehicle hit a mine only 15 or 20 minutes later and that a helicopter flew in to evacuate casualties.

Violence in Afghanistan has risen steadily the last three years as Taliban and other militants step up attacks against international and Afghan troops. President Barack Obama is sending 21,000 additional troops to the country in hopes of improving the deteriorating security situation.