Let Y be a d-dimensional random vector with unknown mean µ and covariance matrix Σ. This paper is motivated by the problem of designing an estimator of Σ that admits tight deviation bounds in the operator norm under minimal assumptions on the underlying distribution, such as existence of only 4th moments of the coordinates of Y . To address this problem, we propose robust modifications of the operator-valued U-statistics, obtain non-asymptotic guarantees for their performance, and demonstrate the implications of these results to the covariance estimation problem under various structural assumptions.