By Asif Shahzad

ISLAMABAD (Reuters) -Pakistan's army has sacked three senior officers, including a lieutenant general, for failing to avert last month's violent attacks on military assets by former prime minister Imran Khan's supporters to protest his arrest, the army's spokesperson said on Monday.

It was a rare announcement as the army usually doesn't make public announcements on internal inquiries and their findings.

At least 102 people are currently being tried in military courts in relation to the violence, Major General Ahmad Sharif Chaudhry told a press conference in the garrison city of Rawalpindi.

He gave no details regarding how many of them were civilians or military officials. He also declined to name the senior officers who had been fired.

Human rights groups have raised concerns over the military trials of civilians, which they say can't ensure a fair defence. They have also been challenged in Pakistan's Supreme Court.

In May, thousands of Khan's supporters rampaged through military installations across the country and vandalized them, including an air base, several military garrisons, the house of a general and the army's headquarters.

Over 5,000 of them were arrested, though most were later released.

"We had to determine why security was breached at army installations," Chaudhry said. "We had to find out what had gone wrong."

He said two departmental inquiries were conducted, headed by major generals, and punishments were given according to their recommendations.

Strict departmental action has also been taken against another 15 army officers, including major generals and brigadiers, Chaudhry said, as part of the army's internal accountability.

The army has said the arson was pre-planned by Khan's party leaders, and have named him in at least two criminal cases as abetting the violence.

Khan, 70, a cricket hero turned politician, has faced a slew of cases since he was ousted from power in a vote of no confidence last year, which he blames on the military's generals, a charge the army denies.

Khan's party has faced a massive crackdown since the May 9 violence.

(Reporting by Asif Shahzad, writing by Shilpa Jamkhandikar; Editing by Toby Chopra and Christina Fincher)