Afghanistan ? A bomb blast killed two U.S. Marines in Afghanistan’s dangerous south, where thousands of American troops have deployed in a massive operation to oust Taliban fighters from the country’s opium poppy region, officials said Sunday.

Some 4,000 Marines moved into Helmand province this month, the largest Marine operation in Afghanistan since the 2001 U.S. invasion. They have met little head-on resistance but remain vulnerable to guerrilla tactics.

“These terrorist attacks are hard to prevent, can be carried out by a few individuals, and do not require a military force capable of confronting the Marines,” said Arturo Munoz, an expert on the tribal environment in Helmand province with the RAND Corp.

The two Marines were killed Saturday in Helmand. Military officials did not release any other details. The military initially reported four Marines had died but later corrected the figure.