UPDATE 1-S.Korea joint investigation unit requests arrest warrant for Yoon over martial law

Changes sourcing in paragraph 1, adds background on Yoon's impeachment trial and refusal of summons in paragraphs 2-7

By Hyunsu Yim

SEOUL, Dec 30 (Reuters) - South Korea's joint investigation unit has requested an arrest warrant for suspended President Yoon Suk Yeol over his short-lived imposition of martial law earlier this month, an official said on Monday.

Yoon has failed to respond to multiple summons for questioning by police and the Corruption Investigation Office for High-ranking Officials who are jointly investigating whether his Dec. 3 martial-law declaration amounted to insurrection.

A court will decide whether to issue an arrest warrant following the request.

Yoon Kab-keun, a lawyer for the suspended president, did not immediately respond to Reuters' requests for comment.

Yoon was suspended from presidential powers after being impeached by parliament earlier this month over his decision to briefly impose martial law.

A Constitutional Court trial is ongoing into whether to reinstate Yoon or remove him permanently from office. It has 180 days to reach a decision.

On Friday, the court held its first preparatory hearing where a request by Yoon's lawyers for a postponement in proceedings to better prepare was denied. The court said it would move swiftly.

The next hearing is due on Jan. 3.

(Reporting by Hyunsu Yim; Editing by Kim Coghill and Michael Perry)

((Hyunsu.Yim@thomsonreuters.com;))