SENJERAY, Afghanistan - Pfc. Sean Provenzano saw it whiz by out of the corner of his eye: a dark object hurled from a rooftop as he patrolled the medieval maze of alleyways in this fort-like walled village at the center of America's Afghan surge.

It bounced off his M-4 Carbine's gun-sight and landed in the dirt a few yards away. At first he mistook it for a rock - kids here often throw them at U.S. troops. But when it rose up and began spinning like a top, he realized it was something far more dangerous.

"GRENADE!!!" the 25-year-old screamed, diving to the ground as the explosion sprayed a deadly burst of shrapnel across the street.

Through a cloud of black smoke and brown dust, Provenzano heard a colleague calling his name. He was alive, unscathed, and incredibly, so was everyone else.

U.S. forces deployed to this village in southern Afghanistan's Kandahar province as part of Barack Obama's troop surge say they came with the noblest intentions: to build up government and security forces, protect the population, make this a safer place. But after a relentless spate of grenade attacks - tossed anonymously over walls and down from rooftops at soldiers patrolling the labyrinthine town - they now keep their distance from the people they're trying to protect.

The change of heart - nine years after the Sept. 11, 2001, terrorist attacks that triggered the war - underscores the profound challenges American forces face in securing this insurgent stronghold, where sympathy for the Taliban runs high and the radical Islamist movement was born in 1994. NATO commanders say a major operation will be launched this month here in Zhari district to clear guerrilla fighters who use the cover of grape vineyards and pomegranate orchards to stage attacks.

"When we first came here, we were giving candy away and water bottles. But as soon as we saw a little kid throw a grenade over the wall, that was it, we don't give 'em anything anymore," said Provenzano, of the 101st Airborne Division's 1st Battalion, 502nd Infantry Regiment.

"We make sure they keep their distance," he said of the population. "You keep 'em away from you as long as you can, because it's only a matter of time before someone gets hurt."

U.S. Defense Secretary Robert Gates visited the joint Afghan-U.S. outpost at Senjeray last week and said he was "encouraged" by signs of progress. But Zhari remains a battlefield where firefights erupt daily. The lush green fields fed by the Arghandab river, just south of the village, are virtual no-go zones controlled by Taliban fighters, and progress in building local governance is painfully slow.

On Wednesday, insurgents ambushed a convoy carrying district chief Kareem Jan, killing one of his guards and getting close enough to steal one of his vehicles. The midday attack on Highway 1 was the third attempt on his life since he assumed office in late May.

Grenade assaults against U.S. forces occur mostly when they move into walled Senjeray. They began in earnest in June, and "a significant amount" of troops have been wounded but none killed, said Capt. Nick Stout, a 27-year-old U.S. company commander from Lake Orion, Mich.

Soldiers say the assaults are aimed at demoralizing or disrupting their operations. Stout said the Taliban or their sympathizers are "trying everything they can to keep us out."

"But you have to continue to get out there, you have to keep them at bay," Stout said. Because "if we don't go in, things could get a lot worse."

Some grenade throwers are "impressionable teenagers" influenced by Taliban propaganda, he said. The youngest is believed to have been 10 or 11 years old.

Troops have captured several, but most escape easily, jumping across rooftops, fleeing through ubiquitous doors, tunnels and passageways hidden inside the sprawling compounds. Others simply blend in with everyone else.

As Provenzano's squad headed into Senjeray on Tuesday accompanied by Afghan police, they scanned rooftops and spread out to lower the risk of multiple injuries in case of an attack.

As light faded from the warren of narrow streets lined with steep brown earthen walls, women and children carrying clay vases of water stepped out of wooden doorways and stared silently. Turbaned men with long beards sat on idling motorcycles, hands crossed, observing the troops as they passed.

Provenzano was at the rear of his squad when the pineapple grenade was thrown at him.

"By the time it skipped off my weapon, I had about three seconds to get as far away as possible," Provenzano said afterward. "If he had thrown it a second later, it would have blown me to pieces, guaranteed."

Whoever threw the grenade was likely long gone.

"It's very, very frustrating, because we're here to help," he said. But "they don't want to be seen speaking to us, because these Taliban come into their homes and say they're going to kill their family if they say anything."