Two German police officers were shot dead on Monday during a traffic stop, police in the southwestern city of Kaiserslautern said, adding that they had launched a massive search for the fugitives.

The two officers, a man and a woman, were shot dead during a routine traffic check at around 4:20 am on Monday on a road near Kusel, a town not far from Kaiserslautern in the German state of Rhineland-Palatinate.

They were able to radio that shots were fired, spokesman Bernhard Christian Erfort told n-tv television.

Reinforcements were unable to help the officers — a 24-year-old woman and 29-year-old man — when they arrived.

Police said they did not know the killers' motives and urged motorists not to pick up hitchhikers, warning that at least one suspect is armed.

The search operation has been extended to the neighbouring regional state of Saarland, not far from the French border, police said on Twitter.

"No description of the perpetrators or the vehicle used for the escape has been established," said the Kaiserslautern police, who also said they did not know in which direction the killers fled.

According to the website of the daily newspaper Bild, the police officers sent a radio message shortly before they were killed, saying they had stopped a suspicious vehicle carrying dead game in the boot.

Erfort said he didn't know whether the officers had seen something particular about the assailants' vehicle that they wanted to check or whether it was just a routine check.

The younger officer killed in Monday's incident was still studying at a police academy, the GdP police union said.