MOSCOW, April 28 (Reuters) - Russian President Vladimir Putin on Friday signed a decree formally increasing the maximum sentence for treason to life in jail, part of a drive to suppress dissent since the start of the war in Ukraine.

The decree was posted on the Kremlin website. Lawmakers had already voted to boost the longest sentences for treason to life, up from 20 years.

Legislators also approved raising the maximum sentence for carrying out "a terrorist act" - defined as a deed which endangered lives and was aimed at destabilizing Russia - to 20 years, from 15 years at present.

Those found guilty of sabotage could also go to jail for 20 years, up from 15, while people convicted of "international terrorism" could be sentenced to life, up from 12 years. The decree did not explain what "international terrorism" is.

Putin signed the new decree at a time when rights groups say authorities are stepping up efforts to quieten the few voices of opposition that remain.

Russia says such laws are required to protect the country from infiltration by Ukraine and Western intelligence agencies.