The company that operates ferry service between the Massachusetts mainland and the islands is warning of disruptions as a nor'easter approaches.

The Steamship Authority says it's adding extra runs to Martha's Vineyard and Nantucket on Tuesday because it anticipates service disruptions Wednesday.

It said it took the step as the National Weather Service issued a high wind watch from 2 p.m. Wednesday to 1 a.m. Thursday, Thanksgiving Day.

Travelers were being urged to check the authority's website for updates.