Japan and the United States have decided to arrange a ministerial meeting to discuss security issues in April without waiting for U.S. President Donald Trump to fill the vacant post of defense secretary, sources with knowledge of bilateral relations have said.

The two countries are rushing to hold the "two plus two" meeting, which is usually attended by foreign and defense ministers from both sides, to lay the groundwork for a summit to be held in May between Trump and Prime Minister Shinzo Abe in Tokyo.

Patrick Shanahan, who has served as the acting U.S. defense secretary following the resignation of Jim Mattis in December, is expected to attend the meeting with U.S. Secretary of State Mike Pompeo, the sources said on Friday.

Japan will be represented by Foreign Minister Taro Kono and Defense Minister Takeshi Iwaya.

Following the abrupt departure of Mattis, Tokyo and Washington had intended to set a date for the next two-plus-two meeting after his successor was picked.

But a U.S. assistant defense secretary who visited Japan earlier this month agreed with Japanese foreign and defense officials that they should not wait further for a Pentagon chief to be appointed, according to the sources.

Trump, who is in favor of making bilateral deals under his "America First" policy agenda, has been eager to sell more U.S. defense equipment to Japan.

Abe, for his part, aims to bolster the Japan-U.S. security alliance amid China's growing maritime assertiveness in the Asia-Pacific region and uncertainty over North Korea's denuclearization plan.