Synopsys VCS gets stuck

Hello, I am facing some weird problem with VCS. I am trying to simulate a simple RTL model with testbench in VCS. It's not possible include all the details of RTL model as its kinda big but in general what happens is, whenever I initialize the clk as '0' in testbench and run the simulation, it gets stuck. I have to force-stop it. I am generating the clock as bellow:

always #1 clk = ~clk;
initial begin
clk = 1;
// delay and driving DUT inputs
end

I am not sure what is happening as I didn't face this problem before. I will appreciate any help in this matter. Thanks in advance.