South Korea's military has said it plans to expand the deployment of an anti-piracy unit now operating off the coast of Africa to the area around the Strait of Hormuz after the United States pressed for help to guard oil tankers.

Attacks on tankers in the Strait of Hormuz off the coast of Iran last year prompted US officials to call for allies to join a planned maritime security mission.

As a key US ally, South Korea has debated the prospect, and the decision to divert a navy unit already in operation southwest of the Arabian peninsula comes as a compromise that will not require fresh parliamentary authorisation.

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While South Korea will deploy its forces to the area, including the Gulf, it will not officially be joining an international coalition of forces, the defence ministry said on Tuesday.

"The South Korean government decided to temporarily expand the deployment of the Cheonghae military unit," a ministry official told reporters.

The step would ensure the safety of citizens and free navigation of South Korean vessels, he added.

The Cheonghae unit will continue with its mission while it cooperates with the coalition, the ministry said, adding that the US had been briefed on the decision, which was also explained to the Iranians separately.

The Strait of Hormuz is a busy passageway into the Gulf, with vessels sailing through it approximately 900 times a year for South Korea, which gets more than 70 percent of its oil from the Middle East, the defence ministry says.

The Cheonghae unit has been stationed in the Gulf of Aden since 2009, working to tackle piracy in partnership with African countries as well as the US and the European Union.

The 302-strong unit operates a 4,500-ton (4,082 tonnes) destroyer, a Lynx anti-submarine helicopter and three speedboats, South Korea's 2018 defence white paper showed.

Among its operations were the rescue of a South Korean ship and its crew in 2011, shooting eight suspected pirates and capturing five others in the incident.