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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 working for 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 hardline regime. American withdrawal is almost unthinkable unless it leaves behind a police and army strong enough to stop the Kabul government from falling in its wake.

While the Marines did eventually secure four officers to accompany them on their night patrol this 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 in the past three years.

"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. "It doesn't. It's just another group of foreigners walking through our neighborhood."

Improving Afghan security forces was high on the agenda when the U.S. and its allies got around to rebuilding the country 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 does not yet have enough police and army units to secure the south. A U.S. government report released in March said more than half the American-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. Officials say the final number needed will likely be much higher. The U.S. also wants to see 134,000 Afghan army troops deployed by the same year.

Building a national security force is especially difficult in a country where people have a scarce sense of national unity after decades of war, much of it fought by armies split along ethnic and tribal lines. Compounding the problem, most police officers have little education and are illiterate.

While salaries have increased in recent years — an officer can expect to earn more than $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 Associated Press. The AP counted 86 Afghan army troops and 868 Afghan police killed in 2008.

Golestan is a hardscrabble valley in southeastern Afghanistan where opium poppy farming is the main business and the Taliban are ever present. There are no paved roads, cell phone service or medical facilities. With no Afghan 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.

The police here have four unarmored trucks, Kalashnikov assault rifles, rocket-propelled grenades and other weapons. But they complain of ammunition shortages. About 30 officers have left the squad in the last 18 months, most citing warnings from the Taliban.

Marines who work with the police in Golestan say those still in the force are generally keen and tenacious, and as sons of the valley, they provide vital intelligence about an enemy most Americans never see. The Afghan translator working with them says the police generally turn down missions not because they are dangerous, but because they could be short on action and hence boring.

The American trainer of the unit that didn't want to go on a patrol in the rain on Sunday spoke to the AP on condition of anonymity, because he was not authorized by his security company to talk to reporters.

Previous attempts to train the police have involved such measures as sending units for eight-week training courses and then embedding instructors with them. The U.S. Defense and State departments have been coordinating American efforts since 2005 and have spent $6.2 billion on the effort, according to the March report by the U.S. Government Accountability Office.

But such efforts have faltered partly because of a lack of mentors on the ground and poor coordination among different agencies and countries involved, the report said.

The U.S. now hopes to recruit more mentors and address other problems as well.

George Earhart, a former police officer from Dearborn, Michigan, broke off his retirement to take a job with the Marines advising them how to train the Afghan force. Many of his answers to questions from a Western reporter about the force begin with the disclaimer: "It is not like in our country."

"We need to get them out, fighting on their own and get the people confident," said Earhart, now six weeks into the job. "It is like an ink spot. It will spread out."

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 wearing combat fatigues, body armor, and modern automatic weapons. But the Afghans took the lead in searching and questioning people.

"We have what we have, and we work with them," said Marine Capt. Robert J. Tart, of New York City. "They are out there, willing to provide security for their own people. To have them with the Marines, that is a big win already."

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

Two weeks ago, police deputy commander Abdullah Tawkalai's house was attacked by about 40 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. Contractors and the cleaner also stay there for fear of Taliban attack.

"I will tell you, if the Marines ever leave this valley, I will go before them," he said, with a laugh.