Garyish UFO in St. Petersburg, Russia

I'm a bit late at reporting this, but better late than never :) I was 7 years old, walking next to the water in St. Petersburg, Russia. At that time, my Grandmother and I noticed a grayish round object floating in the sky. It was almost like a perfectly rounded cloud. But, it was certainly not a cloud. I also noticed a place near by (possibly a jet) and suddenly, the round object started flying (very very fast) toward the plane and then disappeared. The plane continued to fly.
 Now, a 7 year old kid and a Grandma may not seem like a credible source, but my Grandma, at the time, was only 52 years old and she still clearly remembers what happened to this day. I also remember and I'm 100% certain it had to have been a UFO, it flew away Extremely fast.
Witness indicates that the date of the sighting is approximate.  PD