Violence has flared in southern Afghanistan, disrupting a long period of relative quiet brought about by the arrival of large numbers of U.S. troops. In the Sangin District of Helmand province, U.S. Marines who took over from British troops there are finding it hard going, with a heavier casualty rate in their first 90 days than the British suffered in more than three years there. On Saturday, a roadside bomb killed 15 civilians elsewhere in the province. And in Kandahar City, the Taliban assassinated two key officials on Wednesday and Thursday and attempted to kill two policemen, according to Afghan officials. Progress in Kandahar and Helmand provinces had been cited recently by the NATO commander, Gen. David H. Petraeus, and was expected to figure in a White House review of the war due this week. While the assassinations and bombings may turn out to be isolated incidents, the problems in Sangin -- a restive region that the British troops had trouble subduing -- have been more sustained. Since taking over from British troops on Sept. 20, American forces, mostly Marines, have suffered 42 fatalities in Helmand province, according to icasualties.org, an independent website that compiles battlefield data. About 20 of those deaths took place in Sangin, said Maj. Gabrielle M. Chapin, a spokeswoman for the Marines. By comparison, British troops lost at least 76 soldiers in three and a half years there. U.S. military officials acknowledged that Sangin was proving 'a very tough area,' in Chapin's words. 'The 3/5 Marines have not backed down, they have gotten more aggressive, they have taken the fight harder to the enemy,' Maj. Gen. Richard Mills said Friday, referring to the Marines' 3rd Battalion, 5th Regiment. Chapin added that the Marines were expanding the areas they were covering in Sangin. Mills said that 'Sangin is a key crossroads, the last place where the insurgent can grow, harvest and process poppy,' whose cultivation for opium helps finance the Taliban. 'It is the last bit of important terrain in Helmand, and he is fighting hard to hold it.' Mills will soon deploy a new company of heavy tanks, the first American tanks in Afghanistan, in Sangin because of the large numbers of improvised explosive devices there, officials said. 'When the Brits were in Sangin District, they had limited control, but now the Americans are conducting operations all over the district,' said Dawoud Ahmadi, a spokesman for the governor. 'There are casualties to the Americans, but also casualties to the Taliban, including key Taliban commanders.' The bombing in Helmand on Saturday occurred when a truck carrying civilians drove over a mine in the Khanashin District, close to the Pakistani border. In addition to the 15 people killed, four were seriously injured. Ahmadi said the Taliban was increasingly booby-trapping roads used by civilians. In Kandahar, where Taliban assassinations last summer were running as high as four a day, none had been reported in the last month and a half, according to local and military officials. On Thursday, however, Noor Mohammed, the leader of the shura, or council, in the Zhare District, was killed as he headed home from a mosque, according to the district's governor, Niaz Muhammad Sarhadi. On Wednesday, gunmen also killed Muhammed Anwar, the finance officer for the narcotics department of the Kandahar provincial police force. The Taliban claimed responsibility for the attack. Maj. Gen. Muhammad Salim Ahsas, a Kandahar police official, confirmed the assassinations. 'They do weigh on people's minds,' he said. 'We are in a battlefield and are doing our best to minimize risks to officials.' He cited a case a few days earlier in which two attackers who tried to assassinate police officers were killed by the authorities before they could do any harm. 'We are after the enemies; they are on the run,' he said. Others were less optimistic. 'Noor Mohammed was a good friend of mine,' said another member of the Zhare shura, who asked not to be named because he feared for his safety. 'I am so worried now I can't even sleep from fear,' he said. SEVEN AFGHANS KILLED IN PAKTIA KABUL, Afghanistan -- A night operation in Paktia province in eastern Afghanistan ended with the deaths of seven Afghan men and conflicting accounts of whether they were insurgents or civilians. The Paktia police chief, Abdul Ghaafar Safi, said he believed the men were civilians, although he could not rule out that some had ties to the Haqqani militant network, which is allied with the Taliban. Lt. Col. John Dorrian, a NATO spokesman, said the soldiers involved in the shooting were on foot after completing a raid to capture a Haqqani agent. He said they used a bullhorn and a translator to ask in the local language that the men get out of their sport utility vehicle. Instead, one man emerged with an AK-47 and appeared ready to shoot, Dorrian said. After the soldiers shot the man, he said, the other Afghan men opened fire. -- The New York Times 