Active sprint with non-estimated task

Haneen, your question doesn't contain enough detail to comment.

Are these work tasks new to the sprint, but part of an existing sprint item that just weren't accounted for in sprint planning?

Is this new work being pushed to the team during the sprint?

How are other items being estimated?   Relatively, or by time?

What is the best approach when there are couple of work tasks that are not estimated in an active sprint. Is it advisable to estimate it during sprint?

If the work in a backlog is not estimated and kept up-to-date, a team will struggle to forecast how much of it remains. Can you think of any reason to put accurate forecasting in delay?

I'm going to key on this statement.

...when there are couple of work tasks that are not estimated ...

Are these tasks or stories? It sounds like tasks created to inform others on the work that is going to be undertaken. To me estimating task implies the work being undertaken during a Sprint where estimating stories is used to refine Product Backlog Items.  I encourage no PBIs be pulled into a Sprint until it has been estimated because that is an element of agreeing on understanding relative to other PBIs.  Estimating tasks is really up to the Development Team to decide.  They are the ones responsible for accomplishing the work and if they can do it without providing estimates then no one should have anything to say about it.  From the Scrum Guide section that describes the Development Team:

• They are self-organizing. No one (not even the Scrum Master) tells the Development Team how to turn Product Backlog into Increments of potentially releasable functionality;