WASHINGTON • The US military conducted "freedom of navigation" operations against 13 countries and economies last year, including several to challenge China's claims in the South and East China seas, according to an annual report just released by the Pentagon.

The operations were carried out against China, India, Indonesia, Iran, Libya, Malaysia, the Maldives, Oman, the Philippines and Vietnam, the report said, though it did not specify how many such operations were conducted.

Single operations were carried out against the remaining three on the list - Taiwan, Nicaragua and Argentina, the Pentagon said on Monday in the two-page report.

These operations involve sending US 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 number of countries the US challenged last year was down from 2014, when it targeted 19. That was the largest number in more than a decade.

A top US naval official said on Monday that the US and Chinese navies were abiding by a code they set up for unplanned encounters in the South China Sea. Rear Admiral Marcus Hitchcock, the newly minted commander of the carrier strike group led by the USS John C. Stennis, said his ships had been engaged on almost a "24/7 basis" with a "completely professional" People's Liberation Army Navy.

Admiral Hitchcock is among the senior US navy officers urging coast guards to develop a similar code. Security analysts have warned about the increased risk of a clash because of China's growing use of its coast guard in the South China Sea as a de facto navy.

REUTERS, BLOOMBERG