Cambridge police are investigating the circumstances behind four robberies that occurred in three separate neighborhoods in the city in the past week — one involving a gun and a second, a knife — in which some of the victims were forced to give up their phones.

Department spokesman Daniel Riviello said the incidents are “separate and distinct sets of crimes that are believed to be unrelated and geographically diverse.”

They include:

• Monday, a man is assaulted at knifepoint and robbed of his backpack while walking on Garden Street near Cambridge Common shortly before 10:30 p.m. Riviello said “police immediately identified several individuals who may have been responsible,” but they were released when the victim declined to cooperate. Harvard University said the victim was not affiliated with the school.

• Sunday, shortly before 9 p.m. at Buena Vista and Upland Road, a man reports he is approached by two men brandishing a firearm, who rob him of money and his phone. Riviello said the incident “was not immediately reported to police.”

• Thursday at 10:44 p.m., two women in their 20s report they are knocked to the ground at Kirkland and Holden streets by a man. An iPhone is stolen.

• Thursday at 10 p.m., a woman in her 20s walking on Appleton Street is struck in the head from behind by a man, who steals her iPhone.

Though not believed to be related, Riviello said, “the department recognizes these recent crimes may have placed students and residents in fear, and the department is stepping up patrols throughout the city as well as placing teams of officers and plainclothes detectives in affected areas to monitor any suspicious or potentially criminal activity.”

Anyone with information about the robberies is urged to call Cambridge police at 617-349-3300.