Scrum Master Managing or assigning tasks to Team

No. If Scrum outcomes are expected, it would be better to address the actual problem. Why can't they decide what to do? What is the impeding their ability to self-manage the planning of their own work?

Why is the team not ready to select the tasks to work on? Why can't you let them "sit and decide what to do"? It seems like there may be bigger issues here that are preventing the team from being able to self-organize and self-manage around the work that is before them.

Failure is a motivator to learn how to be better.  As Scrum Master you sometimes have to let things go wrong in order for the Scrum Team to realize that there is a problem.  

@Ian Mitchell and @ Thomas Owens are both right.  There are bigger problems than the Developers not being ready to pick up tasks.  As Scrum Master, those are what you should be focusing on. 

This is from the Scrum Guide's section describing Sprint Planning:

Topic Three: How will the chosen work get done?

For each selected Product Backlog item, the Developers plan the work necessary to create an Increment that meets the Definition of Done. This is often done by decomposing Product Backlog items into smaller work items of one day or less. How this is done is at the sole discretion of the Developers. No one else tells them how to turn Product Backlog items into Increments of value.

If were to start assigning tasks, you would be telling the Development team how to turn the Product Backlog item into increments of value.  

This comes from the section that describes the Developer role

The specific skills needed by the Developers are often broad and will vary with the domain of work. However, the Developers are always accountable for:

• Creating a plan for the Sprint, the Sprint Backlog;

• Instilling quality by adhering to a Definition of Done;

• Adapting their plan each day toward the Sprint Goal; and,

• Holding each other accountable as professionals.

Would your developers be living up their responsibilities if you started assigning tasks instead of them deciding who will do what work? 

You said 

As I cant let my team to sit and decide what to do. And we have got lots of work in our backlog.

As a Scrum Master you should be helping them to be self-managing, self-directing.  If you want to help them be productive, help them discover why they aren't able to decide. This is actually removing the impediment they face.  Assigning work is not addressing the actual problem. It is just covering it up. 

As a facilitator, sometimes you have to resist the urge to talk when you ask a question and no one answers. If you wait long enough, eventually someone will start talking. As a Scrum Master, part of your accountability is

Coaching the team members in self-management and cross-functionality;

The more you do for them, the more they depend on you. By assigning tasks instead of coaching them, you are an impediment to the team becoming self-managed. Are the Developers breaking down work in order to answer the How in Sprint planning? This should make the tasks/work more clear and easier to understand what they need to do. Maybe there is opportunity to coach them in becoming cross-functional by doing something like pair programming so they broaden their skills and understand the codebase more? Don't assign or do work. Ask questions.