SHANGHAI—A heavy-duty U.S. icebreaker is steaming toward Antarctic waters to assist a Russian research ship trapped in ice since late last month and a Chinese vessel that got stuck offering support during a continuing drama near the South Pole, authorities said during the weekend.

The U.S. Coast Guard Cutter Polar Star, which had been on its way to Antarctica, will steer toward the Russian vessel Akademik Shokalskiy and China's Xue Long, according to statements from U.S. and Australian authorities. The U.S. Coast Guard said its action comes after requests from the Russian, Chinese and Australian governments.

The entrance of a U.S. vessel, which will need about seven more days to reach the scene, marks the latest international response to an episode that began Dec. 24 some 1,500 nautical miles south of Australia when the Russian vessel, carrying scientists, journalists and tourists, requested help.

After other failed efforts to reach the stranded Russian ship, the Chinese icebreaker Xue Long late last week dispatched its helicopter for a successful rescue of 52 of those aboard the Russian ship. But shortly afterward, Xue Long confirmed that it too was "beset by ice," according to the Australian Maritime Safety Administration, which is coordinating the international response.

Authorities say there is no imminent danger to the Russian ship's 22 crew members and the 101 sailors and scientists aboard China's Xue Long. But thick ice floe complicates prospects to maneuver the vessels, some 21 kilometers from open water.