As five U.S. students are alleged to have discovered, joining Afghanistan's Taliban insurgents isn't easy for outsiders _ one reason the flow of foreign fighters to Afghanistan so far is a small stream compared to the tide once seen in Iraq.

Unlike foreign recruits who found porous borders and easy entry into Iraq, potential jihad fighters heading to the Afghan-Pakistan border face forbidding terrain and closely watched travel routes as well as precision drone attacks. And they are often looked on suspiciously by insular Afghan militant groups who tend to depend on their own tribal members to enlarge their fighting ranks.

U.S. officials say the insurgents' foreign fighter force is growing, but by only small numbers. There's another worry, however: efforts to export the terror tactics to the West.

"The greater concern with Pakistan is that foreign fighters are going there to get their training and leaving. They're going to Germany, Spain or the United States, and that's what threatens the United States in the long term," said Rick Nelson, a counterterrorism expert at the Center for Strategic and International Studies.

While details about the movements of the five detained Americans are still sketchy, Pakistani officials say they tried to join militants in the northwest tribal areas, then travel to Afghanistan to join the fight against U.S. troops.

Pakistani police said the students, ages 18-24, used Facebook and YouTube Web sites to try and connect with extremist groups in Pakistan and are said to have established contact with a Taliban recruiter. Detained last week by Pakistani officials, they have not been charged with any crimes.

Their dashed journey underscores the insular nature of the Pakistan and Afghan militancy.

"The Pakistan Taliban is made up of local militant groups and tribes, and the Afghan Taliban is precisely that, Afghan," said Juan Zarate, who was a senior counterterrorism adviser in the Bush administration. "To the extent there is a foreign dimension to it, it's somewhat of a proving ground for potential onward activity in the West."

Zarate said the Taliban groups, especially those in the Kashmir region, don't need foreign fighters because they've got plenty of local Pashtun recruits who are committed to their cause.

Breaking into those networks often requires a long vetting process.

"It's like if you wanted to join the Mafia, you don't get a meeting with the boss. You have to have someone vouch for you, you have to work through the ranks," said Nelson. In Pakistan, he said, "they're a little more paranoid. Their trust factor is low."