ISLAMABAD (Reuters) - Pakistan's Supreme Court ruled on Wednesday that secret military courts were legal and could pass death sentences on civilians, a judgment that critics said further strengthened the military's grip on power at the expense of civilian authorities.

Military courts were empowered to try suspected militants after Taliban gunmen massacred 134 children at an army-run school in December. The government argued civilians were too scared to convict militants.

Several lawyers challenged the constitutionality of the military courts in the Supreme Court. But on Wednesday, Chief Justice Nasir ul Mulk, announced all "petitions have been dismissed".

A detailed judgment will be released later, he said.

Pakistan, a nuclear-armed nation of 190 million, is plagued by a Taliban insurgency, sectarian violence and militancy.

Military courts have heard at least 100 militants' cases and passed judgment in at least 27, the law ministry said in June. The military published the names of six men sentenced to death in one case.

"The six men convicted by military courts whose execution were stayed by the Supreme Court will now go ahead," Minister of State for Law Ashtar Ausaf Ali told media after the judgment.

There is no public information about the identity of other suspects or convicts, charges or evidence against them, their sentences or appeals. The military has not responded to repeated requests for comment.

Military courts are empowered to try militant suspects until February 2017. The government promised to use that time to reform the broken civilian justice system.

Critics say the government handed the military too much power and there are few signs of reform.

"Ceding space to the military isn't the answer. Parliament can't pass the buck for creating a functioning criminal justice system," said Saroop Ijaz, a lawyer who represents international rights body Human Rights Watch.

"There's been no movement on reform ... When the military courts lapse, the criminal justice system will still be broken."

A Reuters investigation of previous military court martials found that accusations of torture were common.

The military is already holding thousands of civilians without trial, according to Supreme Court hearings into missing persons. It is unclear whether some may face military courts.

"We cannot know what is happening inside those courts, we have no access to them," said Amina Janjua, an activist for the families of the missing whose own husband was detained 10 years ago. "Who will know whether the judges' decision is right and what the proof there was?"