Thailand's King Bhumibol Adulyadej leaves the Siriraj Hospital for a ceremony at the Grand Palace in Bangkok December 5, 2010. King Bhumibol celebrates his 83rd birthday on Sunday. REUTERS/Sukree Sukplang GENEVA (Reuters) - The United Nations human rights office voiced alarm on Tuesday at what it called "shockingly disproportionate prison terms" handed down by military courts in Thailand for insulting the monarchy, and urged authorities to amend the law.

Thai military courts on Friday jailed two people, one for 30 years and the other for 28 years, for insulting the monarchy, the heaviest sentences for the crime in Thai history, lawyers and a legal monitoring group said.

U.N. rights spokeswoman Ravina Shamdasani told a news briefing in Geneva there had been a sharp rise in such cases, with more than 40 recorded since the military took power in a coup in May 2014.

"We also urge the military government to amend the vague and broad lèse-majesté law to bring it in line with international human rights standards," she said. "Until the law is amended, such laws should not be used arbitrarily to curb debate on critical issues of public interest, even when it involves criticism of heads of state or government."

(Reporting by Stephanie Nebehay; Editing by Kevin Liffey)