Will it be a problem if I take a letter of recommendation from a lecturer in my department? I have a pretty well connection with him academically(taught several courses and was one of the teachers of my final year major) and personally. But I am confused if the designation is going to be a problem.

I have requested and managed two from my thesis supervisor and another assistant professor. 
I am planning to apply for graduate studies in the USA.
While a professor would be better, a lecturer should be fine, especially if they hold a doctorate. You want letters from people who know you well enough to make a prediction about your future success, and so a lecturer with whom you have interacted may be a good choice. 

The letters need to speak both about your past accomplishments and about your potential for success. Choose the best people for that. 

I would worry a bit if the person doesn't have a doctorate, however, unless they are also currently in a doctoral program. They need some experience in the situation you are applying for. But a lecturer - current doctoral student probably will work nearly as well. Note that I have a few misgivings about the last statement. Think hard about it if this is the actual case.