In one case in early November two American minesweepers seeking shelter from a storm in the South China Sea were refused permission to enter Hong Kong waters.
The incident prompted an angry reaction from the commander of US navy forces in the Pacific, Admiral Tim Keating, who described China's refusal to grant permission as not "indicative of a country who understands its obligations as a responsible nation".
In the same month a Thanksgiving visit by the USS Kitty Hawk carrier group was also refused as the ship approached Hong Kong.
Keating visited Beijing earlier this month for talks with senior defence officials during which the subject of the port visits was thought to have been discussed.
During the visit Keating urged China to be more transparent about its military objectives, saying the US was paying close attention to the "impressive capabilities" of the growing Chinese military.
Although officials from both sides have not given any details of discussions, Keating's visit to Beijing is likely to have helped smooth relations, if only through the simple fact of him travelling to the Chinese capital.
China has given no formal reason for blocking last year's port visits.
However, comments from Chinese officials and state media reports have hinted at anger over Washington's decision to grant an award to Tibet's spiritual leader, the Dalai Lama, and a decision to sell advanced weapons to Taiwan.
China views the Dalai Lama as what it calls a "splittist", intent on separating Tibet from China, and regards Taiwan as a breakaway province that it will eventually reclaim, by force if necessary.