The contractors were attacked at about 1 a.m. at their desert camp near Jalaw Gir in Kunduz province, 120 miles north of the capital, Kabul.
Mutaleb Beg, the Kunduz police chief, said six to eight assailants killed an Afghan guard at the unfenced camp and then opened fire on the Chinese men as they slept in two tents.
"They died in their beds, most of them with stomach and head wounds," Beg told The Associated Press by telephone after visiting the scene.
Afghanistan's embassy in Beijing identified the 11 slain Chinese, saying nine died on the spot and two more in hospital. Four others who were injured were in stable condition, an embassy statement said.
A spokesman for NATO-led peacekeepers who patrol the area said the toll could rise as reports come in from local clinics.
It was unclear who carried out the attack.
A purported Taliban spokesman, Hamid Agha, said the ousted Islamic militia was not involved. "It doesn't have any link with the Taliban," Agha told The Associated Press by telephone.
Beg said no one was arrested, but raised suspicions about supporters of renegade Afghan warlord Gulbuddin Hekmatyar, who has teamed with the Taliban and vowed to oust the government of U.S.-backed President Hamid Karzai.
The killings are the latest in a string of deadly attacks on relief workers, private contractors and government employees which officials say is an attempt to derail planned September elections. Road workers have been a particularly enticing target for militants.
Last week, three European medical relief workers and two Afghans were killed in an ambush in northwestern Badghis province claimed by the Taliban.
Both Badghis and Kunduz previously were viewed as relatively peaceful, and relief agencies fear militants are expanding their operations from the insurgency-plagued south and east.
A small contingent of German troops are based in Kunduz, NATO's first step in a plan to expand across the north to help provide security for the vote. The rest of the 6,400-strong international force is based in Kabul.
The road contractors worked for the China Railway Shisiju Group, which last year won a World Bank contract to rebuild the highway from Doshi, in neighboring Baghlan province, to the Tajik border.
The $22.5 million project is part of an ambitious plan to restore Afghan infrastructure shattered by more than two decades of fighting. Road workers have also been attacked in the south.
Eight German soldiers stood guard outside the compound housing the Chinese company's headquarters on Thursday afternoon, barring reporters from approaching. Afghan and Chinese officials milled around a squat green building near a fleet of yellow bulldozers and trucks.
The United Nations condemned the "cold-blooded" attack, halted voter registration in Kunduz until at least Saturday and told staff to stay off the roads.
The warning came too late for a team of U.N. counternarcotics officials, whose vehicle was hit Thursday morning by a bomb near Taloqan, the capital of Takhar province.
No one was injured, spokeswoman Sarah Telford said, offering no other details.
The world body has registered about one-third of the estimated 10 million eligible Afghan voters, but has yet to venture into the most insecure areas of the south and east, where U.S. forces have killed dozens of insurgents in recent weeks.
In Beijing, Chinese President Hu Jintao "strongly condemned the inhumane and brutal attack" and urged a swift investigation, Foreign Ministry spokesman Liu Jianchao said.
Jianchao said the firm had 123 workers on the project, and that most had arrived just the day before.
Jianchao said Chinese companies, which are also working on irrigation projects, would not be forced out of Afghanistan but would need better protection.
"China will not give in to any terrorism," he said. "It should be a lesson for us."