Bangladesh's highest court has sentenced to death a leader of the main Islamist party convicted of crimes against humanity during the war for independence from Pakistan in 1971 BBC News reported.

Abdul Kader Mullah of Jamaat-e-Islami, who denied all the charges, was given a life sentence in February. His sentence was increased to death by the Supreme Court as he appealed against his conviction.

Official estimates say more than three million people were killed in the war.

His trial in February sparked protests both from supporters who accuse the government of pursuing a political vendetta and those who saw the prison sentence as lenient.

The assistant secretary general of the Jamaat-e-Islami, Abdul Kader Mullah, 65, was earlier this year found guilty by a war crimes tribunal of five out of six charges, including murder.

He was accused of being behind a series of killings including large-scale massacres in the Mirpur area of Dhaka, which earned him the nickname of "koshai" or butcher of Mirpur, and made him one of the more feared Jamaat leaders.

Prosecutor Ziad Al Malum told AFP that the decision to enhance the sentence was approved by four to one at the court.

Defense lawyer Tajul Islam told the agency he was "stunned" by the verdict. "This is the first time in South Asian judicial history that a trial court sentence has been enhanced by a Supreme Court," he said.

The special court was set up in 2010 by the current Bangladeshi government to deal with those accused of collaborating with Pakistani forces who attempted to stop East Pakistan (as Bangladesh was then) from becoming an independent country.

Bangladesh government figures estimate more than three million people were killed during the independence war, although some researchers put the figure at between 300,000 and 500,000.