GOLESTAN, Afghanistan - The request crackled over the radio in the police station in this remote Afghan valley: four officers needed to accompany U.S. Marines on an overnight patrol.

The appeal was met with little enthusiasm. Though one officer ran for his Kalashnikov, another said he felt sick. His colleague said he was recovering from a long shift the previous day. With rain falling outside, the rest cast their eyes downward to avoid the glare of their commander.

"Come on, you told me you wanted to be warriors," said the unit's trainer, an American with a U.S. security company contracted by Washington to build up the force. "If you want to be in the Afghan National Police, then you follow orders from your commander. If not, you quit."

Securing the help of Afghan security forces is crucial to President Obama's hopes of reversing Taliban gains in Afghanistan eight years after the U.S.-led invasion that ousted the hard-line regime.

American withdrawal is almost unthinkable unless it leaves behind a police force and army strong enough to stop the Kabul government from falling.

While the Marines did eventually secure four officers to accompany them on their night patrol last week, the police force in Golestan offers a window into problems facing police units around the country as they are increasingly expected to join in the fight against the Taliban.

Their commitment will be tested in the coming months as 21,000 new U.S. troops pour into southern Afghanistan - where the Taliban is strongest - to try to turn the tide of an insurgency that has become steadily deadlier.

"When we walk through a town and it is just U.S. Marines and no Afghan forces, if I'm a local, how does that inspire my confidence in the government?" asked Gen. Larry Nicholson, the commander of a brigade of 10,000 recently arrived Marines.

Improving Afghan security forces was high on the agenda when the United States and its allies got around to rebuilding Afghanistan after ousting the Taliban regime in late 2001. Ambitions were high, but - as with so many goals in post-Taliban Afghanistan - progress has been patchy.

Nicholson said he did not yet have enough police and army units to secure the south. A U.S. government report in March said more than half the U.S.-trained units were unable to conduct missions independently.

The Obama administration is aiming for a trained, equipped, and professional force of 82,000 police by the end of 2011, up from about 70,000 now. The United States also wants to see 134,000 Afghan army troops deployed by the same year.

Building a national security force is difficult in a country where people scarcely have a sense of national unity after decades of war, much of it fought by armies split along ethnic and tribal lines. Most police officers are illiterate.

While salaries have risen in recent years - an officer can expect to earn $100 a month - the police force and the Interior Ministry are regarded as two of Afghanistan's most corrupt institutions.

And, as the insurgency began picking up in 2006, Taliban militants began targeting police officers.

Last year, the force lost 10 times more officers than the better trained and equipped army, according to a tally kept by the AP - 86 army troops to 868 police.

Golestan is a hardscrabble southeastern valley where opium poppy farming is the main business and Taliban are ever present. With no army troops in the district, security falls to the police.

The Taliban have threatened or attacked Golestan's 40 or so officers and their families. The officers' former commander is believed to be corrupt and pro-Taliban, and several of his sympathizers remain on the force.

There are judges and a courthouse in the district headquarters, but police make few arrests and hardly any of those make it to court.

On a recent patrol, two policemen accompanied a squad of Marines. Dressed in office shoes and carrying only Kalashnikovs, the police stood out among their American colleagues, who were wearing combat fatigues and body armor, and carried automatic weapons. But the Afghans took the lead in searching and questioning people.

Perhaps the bigger challenge is what happens after the Marines leave.

Two weeks ago, police deputy commander Abdullah Tawkalai's house was attacked by insurgents firing rocket-propelled grenades and automatic weapons. He and others in the house returned fire, keeping the attackers at bay long enough so that they fled, apparently fearing Marines would come to back up the police.

But the militants first shouted a warning, Tawkalai said: " 'We will kill all your family, even the small children.' "

Tawkalai's family has now moved away, and he and the rest of the force are sleeping in the station, a stone's throw from the Marine base.