KABUL, 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 yesterday.

About 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 guerrillas continue to make suicide attacks and place roadside bombs.

"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 Washington-based RAND Corp.

"I would expect the Taliban to avoid pitched battles with the Marines in order to avoid a large number of casualties," he said. "This does not mean they will avoid violence."

The two Marines were killed Saturday in Helmand. Military officials did not release any other details.

The American casualties came after eight British troops were killed in Helmand in a 24-hour period ending Friday, triggering debate in Britain about its role in Afghanistan. Britain has now lost more troops in Afghanistan than in Iraq.

In an interview broadcast yesterday, President Obama called Britain's contribution critically important and said U.S. and British troops face a difficult summer ahead of Afghan elections next month.

"We've got to get through [Afghan] elections," Obama told Sky News. "The most important thing we can do is to combine our military efforts with effective diplomacy and development, so that Afghans feel a greater stake and have a greater capacity to secure their country."

British Prime Minister Gordon Brown has said fighting the Taliban in Afghanistan will help keep extremist groups from staging attacks inside Britain. And he told Afghan President Hamid Karzai in a telephone call yesterday that Britain would stand "shoulder-to-shoulder with the people of Afghanistan for the long haul," according to a statement by the Afghan presidency.

Brown told his troops in an interview yesterday with the British military radio network that it was proving to be a "difficult summer" in Afghanistan but that operations in Helmand were making "considerable progress" toward defeating the Taliban.

But in an editorial yesterday, the Observer newspaper predicted that the British public will soon decide the war is not worth the casualties. "Lives saved by bringing soldiers home will seem a surer benefit than the unproven hypothesis of preventing terrorism with a war thousands of miles away," the newspaper said.

Another American service member died Friday in the United States of wounds suffered in Afghanistan in June, said Lt. Cmdr. Christine Sidenstricker, who confirmed the deaths of the two Marines on Saturday.

The three deaths bring to 104 the number of U.S. troops killed in Afghanistan this year, a record pace. Last year, 151 U.S. troops died in the country. Overall, 193 international troops have died in Afghanistan this year, according to an Associated Press count based on official announcements.

Obama ordered 21,000 additional U.S. troops to Afghanistan earlier this year to help put down the increasingly violent Taliban insurgency.

The troops will help provide security for the Aug. 20 election, and help train army and police units that the United States hopes one day can provide security for the country.

Obama: Probe POW Deaths

President Obama has ordered his national security team to investigate reports that U.S. allies were responsible for the deaths of as many as 2,000 Taliban prisoners during the opening days of the war in Afghanistan.

Obama told CNN in an interview shown yesterday that he doesn't know how the U.S.-allied Northern Alliance behaved in November 2001, but he wants a full accounting before deciding how to move forward.

"If it appears that our conduct in some way supported violations of the laws of war, then I think . . . we have to know about that," he said.

The comments seem to reverse Obama officials' statements on Friday, when they said they had no grounds to investigate the 2001 deaths of Taliban prisoners of war who human-rights groups allege were killed by U.S.-backed forces.

- Associated Press