United boss David Moyes talked with a representative of Kroos' agent during Bayern's win over Borussia Monchengladbach last Friday.
Reports claim United are close to launching a £25million bid for the 25-year-old midfielder.
When asked about reports linking Kroos with a move to Old Trafford, Guardiola said: "He is an important player but things can move fast in football.
"Here today, gone tomorrow."
Kroos' current contract with Bayern Munich is set to expire in 2015.
Kroos said: "I am unable to rule anything out.
"I am under contract with Bayern Munich (until June 2015). Actually there are yet no further talks (regarding a new contract) scheduled. I will therefore fully focus on the second half of the season.
"There are worse situations in life. I play for a top club, and under a top coach. I am pleased, and sometime in the future a decision will be made."