correct grammatically?

“Thousands of Tamils have been massacred by Sri Lakan army, a truth conveniently ignored by UN.”
Is this sentence correct grammatically?

I suppose so.

I would expect to see two definite articles included in that sentence:
“Thousands of Tamils have been massacred by the Sri Lakan army, a truth conveniently ignored by the UN.”

Beeesneees,
“Thousands of Tamils have been massacred by the Sri Lakan army, a truth has been conveniently ignored by the UN.”
Is this correct?
Does it has the same meaning of the former sentence?

“Thousands of Tamils have been massacred by the Sri Lakan army, a truth which has been conveniently ignored by the UN.”

The change of tense obviously changes the meaning. Your first example indicates that it is being ignored now. the second example indicates that it was ignored in the past (and that this possibly changing now).

Beeesneees,

  1. “Thousands of Tamils have been massacred by the Sri Lakan army, a truth which is being conveniently ignored by the UN.”
  2. “Thousands of Tamils have been massacred by the Sri Lakan army, a truth which was being conveniently ignored by the UN.”
    Are these two sentences OK?
    If I omit ‘which’, is the sentence OK or not?

You cannot omit ‘which’ from those sentences.

The first is correct. The 2nd needs a tense change:
Thousands of Tamils were/had been…