JALALABAD, Afghanistan -- As the spotlight of the Afghan war focuses on the south, insurgent activity is increasing in parts of the east, with foreign fighters linked to al-Qaida infiltrating across the rugged mountains with the help of Pakistani militants, Afghan and U.S. officials say.

Security in eastern Afghanistan is critical because the region includes the capital, Kabul, which the insurgents have sought to surround and isolate from the rest of the country. The east also borders Pakistan, where al-Qaida's leaders fled after the 2001 U.S.-led invasion drove the Taliban from power.

Gen. Mohammed Zaman Mahmoodzai, head of Afghanistan's border security force, said infiltration by al-Qaida-linked militants has been increasing in his area since March.

"One out of three are Arabs," he said, coming mostly from Pakistan's Bajaur and Mohmand tribal areas where the Pakistan military is battling Pakistani Taliban insurgents.

The onset of spring makes it easier to move through mountain passes into Afghanistan, though Mahmoodzai thinks the influx of Arabs has been greater than can be explained by seasonal trends.

A NATO official said he thought Mahmooodzai's estimate of Arab infiltration was high, but acknowledged that activity by foreign fighters was running "a little more than average" in the east. He said most of them were thought to be Pakistanis, Chechens and Tajiks.

He spoke on condition of anonymity because the information is sensitive.

In some cases, militants enter the country through legal crossing points such as Torkham, 35 miles east of Jalalabad.

Mahmoodzai said the infiltrators carry fake passports and visas provided by Lashkar-e-Taiba, a Pakistan-based group that India blames for the 2008 attacks in Mumbai that left 166 people dead.

"We know it is Lashkar-e-Taiba because we have sources inside the Afghan Taliban," Mahmoodzai said.

Last month the NATO-led command announced the capture of two Taliban commanders it said were helping Lashkar-e-Taiba members slip into Afghanistan. In reporting the second arrest, a NATO statement referred to a "recent influx" of Lashkar-e-Taiba members into the eastern province of Nangarhar.

The mixture of insurgent groups adds to the complexity of the war in the east, often fought in terrain much more rugged and challenging than in the north or south.

In eastern Afghanistan last year, the U.S. Army pulled out of two outposts in the mountains of Nuristan province after insurgents nearly overran the bases in two battles that claimed 17 American lives.

Insurgents operating from bases in the eastern part of Nuristan are thought to have killed the 10 members of a medical team, including six Americans, gunned down last week in a northern province.