WASHINGTON (Reuters) - U.S. President Barack Obama shortened the prison sentences of 111 convicts serving time for drug-related offenses in his second round of clemency grants this month, the White House said on Tuesday. Obama has now granted a total of 673 commutations during his presidency, more than the number granted by the 10 previous presidents combined, as he seeks to reform the criminal justice system, it said.  For some of the convicts, the commutations mean they will serve only half of their original prison sentences. For instance, Sly Stallone Aikens of South Carolina, serving a sentence of 360 months for using and carrying a gun during a drug trafficking crime, will now serve only 180 months. More than one-third of the 673 convicts had been serving life sentences.     Obama has made reducing the number of people serving long sentences for nonviolent drug offenses a priority. It is one of the rare issues where the president gets support from Republican lawmakers. He launched the clemency program in 2014, inviting thousands of drug offenders and others to seek early release. It was the most ambitious such program in 40 years, but it has struggled under a flood of thousands of unprocessed cases. The White House counsel, Neil Eggleston, said he expects Obama will continue to grant commutations through the end of his presidency, which ends on Jan. 20. But only legislation passed by Congress can “achieve the broader reforms needed to ensure our federal sentencing system operates more fairly and effectively,” Eggleston said.  