The foreign ministry in Colombo said the widespread condemnation, led by the United States and media rights groups, was an attempt at "undermining the independence of the judiciary of Sri Lanka."
J.S. Tissainayagam, 45, who contributed to the local Sunday Times and ran a news website, was found guilty on Monday under the Prevention of Terrorism Act.
He was convicted of receiving money from the Tamil Tigers to fund his work and causing racial hatred through his writings about Tamils affected by the ethnic conflict.
"Questioning or criticising this judicial action is misplaced, especially since Mr.
Tissainayagam has yet the right to appeal," the government said.
US State Department deputy spokesman Robert Wood earlier said that Washington was "disappointed to learn of the verdict and the severity of the sentence.
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In May, US President Barack Obama had cited Tissainayagam as one of several "emblematic examples" of persecuted journalists across the world.
Sri Lanka's