Admiral Mike Mullen, the US military's most senior commander, said on Monday that the first of the 30,000 additional US troops approved by Barack Obama, the US president, would arrive in Afghanistan this week.
"Marines from Camp Lejeune will arrive this very week," he said, referring to the largest US Marine Corps base on the US east coast.
A 1,500-strong Marine contingent is expected to be deployed in southern Helmand as part of a vanguard set to prepare the logistics for thousands more due in the coming months.
Mullen, who is visiting Afghanistan for talks on the new strategy, warned that violence is likely to get worse before it gets better and said anti-government fighters had the upper hand in nearly one-third of Afghan provinces.
"I told our troops heading here to steel themselves for more combat and more casualties," he said.
"The insurgency has grown more violent, more pervasive, more sophisticated. Insurgents have dominant influence in nine of Afghanistan 34 provinces."
Mohammad Akbar Barakzai, the provincial governor in Baghlan, said the policemen killed there were "on duty to provide security for the military convoys and local people in the main highway in Baghlan province".
Barakzai said the police post that came under attack in Baghlan Markazi district had been set up recently to protect convoys supplying international troops.
A man claiming to speak for Hezb-e-Islami, an Afghan group, told the AFP news agency in a telephone call that his group had carried out the attack in Baghlan.
One of the group's fighters also died during the assault, he said.
Hezb-e-Islami is led by former Gulbuddin Hekmatyar, a former Afghan prime minister who is on the United States' most wanted list of "terror" suspects.
A provinical government spokesman said that the attack in Helmad was carried out by their colleagues of the police officers killed.
"Three of them were involved in the plot. They fired at their police colleagues killing seven. One of the three grabbed weapons and a police vehicle and managed to escape to the Taliban," Daud Ahmadi told the AFP news agency, without explaining the discrepancy in the death toll.
"Two others were hiding in nearby areas and were detained during a search operation in the morning," he said.