Trouble connecting Xbee S1 to USB port of Raspberry Pi 3

I have a Xbee S1 connected to the USB port of a Raspberry pi 3 and digi.xbee.devices package (python package)is used to set up communication. The communication works fine as long as I have the Xbee connected to the serial port but for some reason it does not work properly with USB, that is the connection is intermittent(the device goes to AT mode in between communication). However, if I check the device mode in XCTU it shows API.

The Xbee sends data for a small period of time if I disconnect the cable and replug it again.

I tried using n USB shield thinking that the unreliable transmission is due to loose connection in the cable, but that also did not work. I also tried connection the Xbee with the laptop and it shows the same error.

Any help or suggestions that you could offer will be of great help to me.