U.S. National Security Adviser John Bolton held a series of meetings with top Japanese officials Monday, with Washington floating a plan for a multinational coalition to safeguard shipping in the Middle East amid rising tensions with Iran.

Bolton and Foreign Minister Taro Kono discussed Iran, a government source said, without providing further details. The United States is asking allies to contribute personnel, naval vessels and funds for patrols in the Strait of Hormuz, the sea lane connecting the Persian Gulf and the Gulf of Oman through which more than a fifth of the world's oil passes.

Bolton also met separately with Shotaro Yachi, a close aide to Prime Minister Shinzo Abe and head of the secretariat of the National Security Council, and Defense Minister Takeshi Iwaya.

"We had very productive discussions across a range of issues that are important from a security point of view to both the United States and Japan," Bolton told reporters after the meeting with Iwaya.

"The minister and I discussed how to strengthen the alliance going forward in a variety of ways and I'm sure those discussions will continue over the next days and weeks."

Abe said he would consider what role Japan can play in the Middle East, a region the resource-poor country heavily relies on to meet its energy needs.

"We need to take a close look at the U.S. plan, at what kind of measures will be necessary," he said at a press conference on the results of Sunday's upper house election.

Related coverage:

Japan defense chief has "no plan" to dispatch SDF to Middle East

Japan will "play its role" in de-escalating tensions between the United States and Iran, he added. Japanese government officials say it is unlikely that the Self-Defense Forces can be deployed without new legislation to allow it.

Tensions between the United States and Iran have heightened since President Donald Trump's decision to withdraw from a 2015 nuclear deal between Iran and major world powers, and recent attacks on oil tankers near the strait that Washington has blamed on Tehran.

Bolton and Kono also discussed tensions between Tokyo and Seoul amid a dispute over wartime labor and Japan's decision to impose restrictions on South Korea-bound exports of materials used to manufacture semiconductors and display panels for smartphones and TVs, according to a Japanese government official.

Trump has expressed his readiness to mediate between the U.S. allies if asked.

Bolton will head to South Korea on Tuesday.