BOSTON -- Boston University police warned students to remain vigilant and travel in groups at night after a knifepoint robbery outside a residence hall on Saturday morning left students rattled.

The robbery, which occurred outside of Rich Hall at 277 Babcock St., follows another armed holdup inside a Northeastern University residence hall on Thursday and the mugging of a Harvard professor near the school's campus earlier this week.

In the BU incident, two female students told police they were approached by a white male in his 20s carrying a knife outside of the dormitory at about 4:15 a.m. The students, both 18, said the man stole $50 and a pack of cigarettes from them.

“Obviously these two individuals were targeted,” said an undercover police officer at the school. “It is very, very unusual that somebody would come up onto that plaza and do something like this. There is always a police presence in that area.”

BU police plan to increase their patrols around the school’s west campus area this weekend following the robbery.

“I don’t think I’m going to ever let one of my friends walk home alone again,” said a BU student who lives in a dormitory near Rich Hall.

Most colleges and universities around Boston have escort security services, and police are telling students, faculty and guests that they should use them if they feel unsafe traveling around campus.