NEW DELHI (AFP) - India's Supreme Court on Friday cleared the way for the execution of a Sikh militant, rejecting his appeal in a ruling that could lead to more death sentences being carried out.

Devender Pal Singh Bhullar from northwestern Punjab, who was convicted over a car bombing in New Delhi in 1993, had appealed for his sentence to be commuted to life imprisonment on the grounds that he had spent two decades on death row.

He also challenged the right of the state to execute mentally ill convicts, claiming he had developed psychological problems while languishing in prison, including schizophrenia.

A Supreme Court bench ruled that neither his lengthy wait in prison nor his apparent mental problems were reasons to set aside the death sentence.

"The petitioner has not been able to make out a case for commuting his sentence," Justice G.S. Singhvi said, reading out the judgement.

The ruling was being followed by more than a dozen other prisoners whose executions have also been held up for decades, including three men convicted over the assassination of former prime minister Rajiv Gandhi in 1991.

Indian courts hand down death sentences for the "rarest of rare" crimes but the country had not carried out an execution for eight years until last November when it put to death the only surviving gunman from the 2008 Mumbai attacks.

Another militant convicted for his role in an attack on the Indian parliament in 2001 was executed in February after his appeal for clemency was rejected by President Pranab Mukherjee.

Mr Mukherjee, who took office in July last year, has signalled a hard line on the death penalty by regularly dismissing mercy pleas - unlike his three predecessors.

More than 400 people are on death row in India.