The U.S. military conducted "freedom of navigation" operations against 13 countries last year, including several in dispute of China's claims in the South and East China seas, according to an annual Pentagon report released on Monday.
The operations were against China, India, Indonesia, Iran, Libya, Malaysia, the Maldives, Oman, the Philippines and Vietnam, the report said.
It did not specify how many such operations were conducted against each of those countries. The U.S. military carried out single operations against Taiwan, Nicaragua and Argentina, for a total of 13 countries, the department said in the two-page report.
The freedom of navigation operations involve sending U.S. Navy ships and military aircraft into areas where other countries have tried to limit access. The aim is to demonstrate that the international community does not accept such restrictions.
The U.S. military has repeatedly conducted operations disputing China's maritime claims in recent years and did so again in 2015, a year in which Beijing's island-building activity in the resource-rich areas of the South China Sea led to rising tensions in the region.
A U.S. guided-missile destroyer conducted a freedom of navigation patrol near one of China's man-made islands in the Spratly archipelago in October. U.S. military flights near the islands have been warned to go away.
U.S. Defense Secretary Ash Carter said the Navy would continue to operate in the region despite China's condemnation of the patrols.
Admiral Harry Harris, the head of U.S. Pacific Command, said this year the Navy would step up the freedom of navigation operations in the South China Sea because of concerns China is attempting to assert its dominance by building military facilities there.
U.S. freedom of navigation operations last year also challenged China's claims of jurisdiction in the airspace above its maritime Exclusive Economic Zone as well as restrictions it has tried to impose on aircraft flying through an Air Defense Identification Zone over the East China Sea.
The number of countries the United States challenged last year was down from 2014, when it targeted 19 countries. That was the largest number in more than a decade.
Iran and the Philippines have been the most frequently challenged countries over the years, mainly because they sit astride heavily traveled sea lanes whose use they have tried to limit or govern.