Rights groups including Amnesty International and Human Rights Watch Tuesday called for a UN investigation into last year's port blast in Lebanon's capital Beirut in light of a stalled domestic probe.

The August 4 explosion at Beirut port killed more than 200 people and destroyed swaths of the capital but ten months on, little light has been shed on the circumstances that led to Lebanon's worst peacetime disaster.

Despite growing calls at home and abroad for an impartial investigation, Lebanese authorities have repeatedly said they reject an international probe.

"As we approach the one-year anniversary of the explosion, the case for such an international investigation has only strengthened," said a joint letter signed by 53 Lebanese, regional, and international rights groups in addition to survivors and families of the victims.

The UN's "Human Rights Council has the opportunity to assist Lebanon to meet its human rights obligations by conducting an investigative or fact-finding mission into the blast," the letter said.

One of the largest non-nuclear blasts in history was caused by a vast stock of highly explosive ammonium nitrate that had sat for years in a port warehouse, little more than a stone's throw from residential districts.

The lead investigative judge on the case said this month that he will soon start interrogating suspects after completing a preliminary phase of investigations.

He is looking into whether the blaze that caused the blast was sparked by accident or deliberately, without ruling out the possibility of a foreign attack.

A recent report submitted by French investigators assisting in the local probe dismissed the likelihood of such an attack, a judicial source told AFP.

"The continuing failure of the domestic process reinforces the need for an international investigation to determine the causes of the explosion and who was responsible," HRW said in a separate statement.

"The cost of such a failure includes not just the absence of justice for victims, but the... risk of further abuse and negligence by the responsible parties."

With the anniversary of the blast approaching, Lebanese leaders are under growing pressure at home and abroad to provide answers.

Many citizens blame the blast on decades of negligence and corruption by these same leaders -- none of whom have been detained over the tragedy.

"The Lebanese authorities have obstructed, evaded, and delayed the ongoing domestic investigation," Amnesty said in its own statement.

"The Human Rights Council must establish an investigative or fact-finding mission into the blast to identify whether conduct by the state caused or contributed to unlawful deaths."