KABUL, Afghanistan - Insurgents exchanged fire with U.S. troops aboard a Black Hawk helicopter in central Afghanistan on Monday before the aircraft was hit and forced to land. The crew was rescued, but in the north a suicide bomber killed two U.S. soldiers.

Lt.-Cmdr. Walter Matthews, a U.S. military spokesman, said there were no U.S. casualties as a result of the crash in a province neighbouring Kabul.

"The helicopter crew exchanged fire with the enemy before the damage brought the helicopter down," Matthews said.

At least four militants were killed in the exchange, said Fazel Karim Muslim, the chief of Sayed Abad district.

Another helicopter hovered as the U.S. troops secured the area around the downed chopper, which didn't appear to sustain major damage, Muslim said.

The United States and other foreign forces rely heavily on helicopters for transportation around Afghanistan, which is covered by rough mountains and long stretches of desert and has few decent roads. Insurgents rarely bring down military helicopters, though they have hit several in recent years.

Wardak province has seen an increase in insurgent activity the last two years, and its main highway is now extremely risky to travel on, particularly at night. In mid-October, a U.S. Special Forces raid freed a kidnapped American working for the Army Corps of Engineers who had been held captive in Wardak for two months.

Also Monday, a suicide bomber wearing a police uniform blew himself up at a police station in northern Afghanistan, killing two American soldiers and wounding five other people, including an American, officials said.

The bomber entered a police station in Pul-e-Khumri, capital of Baghlan province, while Afghan officials were meeting with U.S. troops advising a police training program, provincial police chief Gen. Abdul Rahman Sayed Kheil said.

The blast killed two American soldiers who had been beside a beige Humvee, AP Television News footage of the blast scene showed.

It was not immediately clear if the bomber was a policeman or just wearing the police uniform.

Zabiullah Mujahid, a Taliban spokesman, claimed responsibility for the blast in a phone call to an Associated Press reporter. Mujahid said the bomber's name was Abdul Had and that he was from Baghlan province.

Militants in Afghanistan have in the past disguised themselves in police or army uniforms when attacking Afghan and foreign troops. But actual policemen in the Afghan force were responsible for at least two recent attacks in eastern Afghanistan in which two U.S. soldiers died after police opened fire on them.

More U.S. and NATO troops have died this year in Afghanistan than any other year since the 2001 U.S. invasion, in part because Taliban militants are launching increasingly complex and deadly attacks.

But NATO's top commander in Afghanistan, U.S. Gen. David McKiernan, said he is tired of negative headlines and what he sees as a wave of unwarranted pessimism in news reports.

"Somebody likes to report an attack somewhere and that becomes the trend in Afghanistan, or they don't report the positive events or the absolute brutality or the illegitimacy of the Taliban," McKiernan told The Associated Press in an interview Sunday.

"What happens sometimes in reporting is that there's this idea that the Taliban is at the gates of Kabul, or after Sarposa (a massive June prison break) they're about ready to take control of Kandahar, or they're resurgent in Uruzgan or Helmand, and it's just not true," he said.

McKiernan, who took command of the NATO mission here in June, has acknowledged that the country lacks security and governance in many regions but concluded in a recent news conference that "we are not losing Afghanistan."

Elsewhere, the Interior Ministry said Taliban militants kidnapped 17 road construction workers in Kunar province on Sunday. The ministry said the kidnappers were gunmen for a Mullah Nasrullah. Three of the workers had already been released, it said. Kidnappings by militants and criminal groups seeking ransom is a growing problem in Afghanistan.